## 1. Introduction



Leibniz and his follower Christian Wolff were two of Kant's most
cited references. At one time or another, Kant addressed all of
Leibniz's main doctrines, including his defense of living forces
against the Cartesians, his attack on absolute space and time against
the Newtonians, his immaterial atomism or monadology, his theodicy,
and his various principles and laws -- the identity of
indiscernibles, continuity, non-contradiction and sufficient reason.
The challenging tone towards the followers of Leibniz that the young
Kant, as an aspiring physicist and cosmologist, and later as a strong
advocate of free will, adopted gave way in time to a more respectful
engagement with the philosopher himself, as Kant became an
increasingly determined critic of materialism.



Yet Kant's attitude towards his famous predecessor, whose ideas
were extensively discussed in French, and later in Prussian
intellectual circles after Leibniz's death in 1716, never
reaches equilibrium. This is unsurprising, for Kant's own texts
show an attraction to teleological theories of the whole, as well as
to epistemological caution. Kant describes Leibniz as one of the
greatest and most successful reformers of the modern era (9:32), along
with John Locke, and as a genius, along with Isaac Newton (7.226). Yet
he also refers repeatedly to the errors of the Leibnizians, whom he
considers "dogmatic" philosophers. Their conviction that
human reason could acquire knowledge of supersensible entities,
including the soul and God, necessitated, in Kant's view, a
"critique." Locke had attempted to construct a critical
philosophy against the Cartesians but had failed to carry his
programme through. At the same time, the attacks of Locke and Hume on
metaphysics had "opened the door to *Schwaermerei*". So
determined were Kant's contemporaries to find affirmations of
immortality and divine justice, that, he believed, many had turned to
the occult. "So much there was deceptive, that it is necessary
to suspend the whole enterprise and to bring instead the method of the
critical philosophy into play. This consists of examining the process
of reason itself, of dividing and testing the human knowledge capacity
in order to determine how far out its boundaries may be placed"
(9.32).



The suspension of the whole enterprise announced by Kant should not
obscure the fact that Leibniz and Kant shared an ethico-religious
conception of philosophy. Leibniz wrote in an era in which the
universities were still dominated by Christian philosophy. Whatever
his private leanings towards mystical and philosophical religion, he
believed that a strong and unified religious authority was essential
to the maintenance of moral and political order, and the content of
morality did not strike him as problematic. Kant, though steeped in
Protestant theology and moral philosophy, favoured the newer trend
towards the academic autonomy of philosophy and secular morality and
governance. Yet he had to contend with the rising influence of
materialism and deterministic psychology in Germany (10:145); the
sentiment- and convention-based accounts of morality and moral
motivation of the British school, the attacks on metaphysics and
conventional morals of Helvetius, La Mettrie (19:109), and
Voltaire (15:336) and the attacks of Hume on religion and the concept
of the soul. How could morality be reconciled with Newtonian science
and how could the existence of ineluctable duties be established in
the face of the variety of human practices and customs established by
Montesquieu and the numerous travel writers on whose accounts Kant
relied in compiling his lectures on Anthropology? Metaphysics could
not provide knowledge of the supersensible, including the existence of
God, the possibility of the realization of a highest good in the
natural world, or the ability of human beings to realize that good
through their endowments. Human aspirations in this regard were
transcendent, futile, and "entirely vacuous" (20:
301).



For Leibniz, the Kingdom of Nature and the Kingdom of Grace were
parallel orders. Everything in nature happened, as Hobbes and Spinoza
maintained, on account of 'mechanical' principles. At the
same time, Leibniz insisted in his *Principles of Nature and Grace
based on Reason* (1714):



> 
> [A]ll minds, whether of men or [spirits], entering into a kind of
> society with God, by virtue of reason and eternal truths, are members
> of a City of God, that is members of the perfect state, formed and
> governed by the greatest and best of monarchs. Here there is no crime
> without punishment, no good action without proportionate reward, and
> finally as much virtue and happiness as is possible. (Ariew and
> Garber, 212)
> 



Referencing Leibniz, Kant adapted this dualistic scheme into his own
scheme of transcendental ideas. In nature, everything happened accord
to laws, though the course of the world might depend on a somewhat
richer ontology, including lawfully-acting generative forces for the
formation of plants and animals, than Leibniz had allowed for, forces
that human beings might never really understand. At the same time,
rational beings, though not other souls or minds, formed a moral
community (see Guyer).



> 
> To view ourselves, therefore, as in the world of grace, where all
> happiness awaits us, except insofar as we ourselves limit our share in
> it through being unworthy of happiness, is, from the practical
> standpoint, a necessary idea of reason. (KRV A812/B840)
> 



In this community, reward for having lived a morally worthy life in
the form of everlasting happiness was intrinsically deserved, but this
could only be hoped for and kept in view, not proved (KRV
A810-18/B841-46).



Kant denied that the present world contained as much goodness and
happiness as possible. Rather, it was definitely progressing towards
greater cultural development, and perhaps moral development. Hope and
effort, especially in the latter struggle where they were most needed,
was itself morally mandated (8:8-32). So the 'idea of a moral
world' had 'objective reality' as a 'corpus
mysticum of the rational beings in it' even if it was only the
sensible world viewed in a different but obligatory way.



The dogmatic metaphysicians preceding him, Kant argued, imagined that
they could demonstrate the truth of their doctrines in rigorous
mathematical fashion, but metaphysical concepts lacked the precision
and intelligibility of mathematical concepts. At the same time, they
put their trust in intellectual intuition, which was no more certain
than the visionary and mystical writings of Emmanuel Swedenborg, who
had set down his hallucinations and angelic dictations in twelve
volumes of the *Arcana Coelestia* (1749-56), which Kant
read in 1765 and satirized in his *Dreams of a Spirit Seer* in
1766. Leibniz's monadology exemplified both errors, for Leibniz
thought that, merely by rationally considering the problem of the
divisibility of matter, he could demonstrate that the basic
constituents of the universe were not material particles obeying the
laws of mechanics, but living or at least minded beings with
perception and appetite, acting for the good of the universe. At the
same time, the details of his picture of the world behind the
appearances, incorporating slumbering monads, confused omniscience,
and the pre-established harmony, seemed to Kant gratuitous
fancies.



Where Leibniz rescued ethics and religion by claiming to discover a
hidden reality of immortal, spontaneous souls forming a Kingdom of
Grace beneath the material and causally-determined appearances, Kant
believed he could accomplish the task of reconciling the scientific
view of the world with moral aspiration and accountability by
investigating the necessary preconditions of our experience. Necessary
forms of thought such as space, time, causality and objecthood would
thereby be distinguished from constraints embedded in reality. Though
materialism is commonly associated with French Enlightenment figures,
Kant considered it an English vice as well. He associated it not
however with Hobbes and Locke but with Joseph Priestley (KRV B 773;
4:258), and he was most certainly aware of its German development
through Ernst Platner and others (see Rumore). Determinism and
materialism seemed to him to support a lax moral philosophy, in which
pleasure was conceived as the summum bonum, morality was conventional,
and humans were anyway machines devoid of responsibility for their
actions. In those not given to dissipation, skepticism and empiricism
led, Kant thought, to a sense of moral futility, misanthropy and
despair. Kant was determined to attack fashionable, pessimistic, and
libertine philosophies, but he had to show that he rejected
rationalist demonstration as thoroughly as any empiricist.



His manner of doing so was exceedingly elegant. Kant did not merely
challenge the logic of, or poke fun at the extravagances of
Leibniz's metaphysics, though he is not above a joke regarding
the monads -- potential human lives -- he might be
swallowing with his morning coffee (2:327). In the antinomies section
of the *Critique of Pure Reason*, he shows that, for every
"proof" of an important metaphysical proposition, such as
determinism, atomism, or the eternity of the universe, a proof of the
contrary proposition, such as the existence of exceptions to
mechanical causality, infinite divisibility, or the temporal finitude
of the world can be supplied. By attempting to prove too much, human
rationality accomplished too little. Traditional metaphysics left
reason perturbed, confused, and unfulfilled.



As many of Leibniz's writings were not published until the
19th or 20th century, an accurate assessment of
the relation between the Leibniz and Kant can take into account only
those works in circulation in the second half of the 18th
century and available to Kant, his teachers and his interlocutors.
These comprised the *Meditations on Knowledge, Truth and
Ideas*, the *New System of the Nature and Communication of
Substances*, the *Specimen Dynamicum*, the
*Theodicy*, the *Monadology* and the *Principles of
Nature and Grace*, the *Leibniz-Clarke Correspondence*, and
the posthumous *New Essays*, which Kant read four years after
their publication, in 1769. A collection of diverse pieces, edited by
Pierre Desmaizeaux was published in 1720; followed by the more
comprehensive *Oeuvres philosophiques* edited by Raspe
in 1765 , and the *Opera Omnia* issued by Dutens in 1768.
Christian Wolff, who may have had privileged access to some of
Leibniz's unpublished writings, wrote a series of textbooks
under the title *Vernunftige Gedancken* beginning in 1719,
reformulating Leibniz's scattered thoughts on atomism,
determinism, pre-established harmony, and theodicy into scholastic
format, that served to bring these doctrines into prominence, as did
some writings of Alexander Baumgarten and G.F. Meier, Euler's
*Letters to a German Princess* (1768-1772), and Emilie du
Chatelet's *Naturlehre an Ihren Sohn* which deal with
major Leibnizian themes, including substance, soul-body relations, and
the problem of evil. If Kant brooded on Leibnizian topics, however, it
was not, except perhaps in the period of the controversy with J.A.
Eberhard, in which Kant was concerned to distinguish Leibniz from his
followers and to champion what he saw as Leibniz's special
Platonic contributions, with an eye to understanding Leibniz's
system as a whole or extracting the best possible interpretation from
it, but rather with an eye to avoiding his errors.



Insofar as Kant professed embarrassment about his pre-1770 essays, it
might be tempting to divide his discussion of Leibniz into a
pre-critical (before 1770) and a critical phase. Yet this division is
not specially illuminating, and recent scholars have questioned the
formerly standard periodization (see the entry on
 Kant's philosophical development).
 *The True Measurement of Living Forces*, (1747), the
*Physical Monadology* (1756), *New Elucidation* (1755),
the *Essay on some Treatments of Optimism* (1759), *Negative
Quantities* (1763) and the *Dreams of a Spirit-Seer* (1766)
are all technically "pre-critical," but they are critical
with respect to logical, physical, and theological Leibnizian
principles and doctrines. It is plausible to see the "silent
decade" between 1771 and 1780 as the interval in which Kant
decided how to manage the skeptical problem he now saw as threatening
morality, arriving finally at the positive philosophy in the
*Critique of Pure Reason* (1781) which steers (perhaps only
temporarily, judging by the *Opus Postumum*) a middle course
between visionary and mystical enthusiasm and skepticism. Kant's
further thoughts on Leibniz were developed in the *Metaphysical
Foundations of Natural Science* (1786), the polemical essays,
*On a Discovery According to which any Critique of Pure Reason has
been made Superfluous by an Earlier One* (1790) and *What
Progress has Metaphysics Made in Germany since the Time of Leibniz and
Wolff*?(1791), and in the *Critique of Judgement*
(1790).


## 2. The Principle of Contradiction



Leibniz gives various formulations to his Principle of Contradiction
or Law of Identity, but the central idea is that a proposition and its
negation cannot both be true (G 7: 299). Leibniz hoped to be able to
construct a logical calculus that would enable all significant truths
to be demonstrated, since every concept must include, be included in,
or exclude every other. A concept such as 'human,' he
maintained, included the concepts 'animal,'
'rational,' 'bipedal,' etc., so that a true
statement such as 'Humans are animals' was true in virtue
of the inclusion of the predicate in the subject. Though the magnitude
of Leibniz's contribution to combinatorial mathematics and logic
remained unknown until the twentieth century, Kant offers some
skeptical remarks to what he takes to be the Leibnizian programme in
Section II of the *New Elucidation*. Later, he offers two
specific criticisms of the Principle of Contradiction taken not in a
logical, but in an ontological sense:



First, Kant claims, the Principle is too weak to ban nonentities from
theories. In Sect 28 of the *Inaugural Dissertation* (1770), he
complains of "fictitious forces fabricated at will, which, not
finding any obstacle in the principle of contradiction are poured
forth in multitudes by those of speculative mind." These
fictitious forces presumably included extrasensory powers of
perception and the direct action of souls on souls. Second, the
Principle of Contradiction is too strong. Leibniz's Neoplatonic
leanings induce him to see creatures as fragments of the divine, whose
imperfections are mere lacks. There are, in his ontology, no beings or
forces opposing God (*Theodicy*, SS20). Kant's
orientation is more Manichaean; he thinks that acceptance of the
Principle leads the theorist to underrepresent the extent of conflict
in the world and its constructive aspects. Kant insists that opposing
forces, "hindering and counteracting processes" operate
ceaselessly in nature and in history. The opposition of attractive and
repulsive forces in physics produces the phenomena of matter (4: 508
ff); the opposition of good and evil principles in the human soul
produces morality, (6:1-190); and antagonism and conflict in
geopolitics produce peace and progress (6:24). Kant denies the
Leibnizian claim that all evil follows from the limitations of
creatures (KRV A 273/B. 329).


## 3. The Identity of Indiscernibles



The Leibnizian principle that "there are never two things in
nature which are exactly alike and in which it is impossible to find a
difference that is internal or founded on an intrinsic
denomination" is enunciated in the *Monadology* (G 6:
608), as well as in the *Correspondence* with Samuel Clarke (G
7: 372). Leibniz had abandoned his earlier view that two entities
could be distinguished by place alone when he came to his view of real
substances as infinitely complex and unique and space as ideal. Kant
found the Principle arbitrary. To insist that any two objects
presented to us in experience must be qualitatively different in some
respect was, Kant said, to take appearances for intelligibilia (KRV A
264/B320). We cannot have two concepts -- concepts of two things
-- that are alike in all their specifications, but we can
certainly have two empirical objects that are exactly alike. Why
should we not be able to imagine two identical water droplets?
(20:280). It is sufficient for there to be two that they present
themselves to us (veridically) in our visual space as two.
Leibniz's blunder on this score was, for Kant, an indication
that Leibniz had failed to grasp an important feature of sensory
experience, namely that it, unlike thought in general, is always
spatial.


## 4. Substance and "Matter"



Leibniz's metaphysics was developed within, and in part as a
reaction to the mechanical philosophy of the mid-to-late 17th century,
revived by Galileo, Descartes, Gassendi, Boyle, Newton, and Locke.
While the term "substance," meaning the indestructible
stuff of the universe, was retained by Descartes in his discussions of
*res extensa*, the mechanical philosophers adopted a
corpuscularian theory in which objects were temporary aggregates of
solid, indestructible particles with various figures and motions, and
most though by no means all change occurred through their contact,
pressure, collision, entangling, and so on. Leibniz contested the
corpuscularian image, insisting that it was insufficiently profound
and inherently self-contradictory (G 4: 480), and holding that matter
was a 'phenomenon' founded upon the reality of
"metaphysical points" or, as he later termed them, monads:
qualitatively unique,indestructible and indivisible units that also
perceived and strove (G 6: 608).



Kant accused Leibniz, along with Spinoza, of 'taking appearances
for things in themselves' (KRV A264/B320). One can see his
point with respect to water droplets as noted, but it is nevertheless
a somewhat mixed-up claim. As Kant understood him, in failing to
distinguish between intellectual representation and sensory
perception, Leibniz believed we saw aggregates of monads as empirical
objects. But Leibniz distinguished sharply between the everyday
perceptual world of living beings and inanimate objects and the
invisible monadic order. Unextended monads were non causally
interacting but enjoyed (in a confusedly omniscient manner) the
universal pre-established harmony of perceptions, while empirical
humans perceived individually subvisible empirical objects such as
animalcula and fine powders through causal mechanisms in a confused
manner (GM III:546-7; AG IV:426). But Leibniz was not clear about how
the unextended monads projected into a visible world and neither were
his later commentators. After his critical turn, Kant had decided that
"things in themselves" that compose external reality are
not perceived at all. They are not in causal contact with us, though
they affect us in such a way that we experience a sensory world
structured according to the categories of time, space, causality and
objecthood. The impact driven, push and pull of the appearances
therefore did not, in Kant's view, preclude the possibility of
the origination of free actions by a noumenal subject. 



Matter, Kant could readily agree with Leibniz, cannot be a thing in
itself, stuff possessed of characteristics and qualities independent
of human perception; what we call matter is an appearance (4:507). The
true nature of mind-independent external reality cannot be described
by reference to shape, contact or movement, which characterize only
the objects presented to us (A265-6/B 321-2). He understood
Leibniz's reasoning in favour of monads as follows: It is
impossible to *conceive* two material atoms as both different
from one another and as simple, i.e partless; yet possible to conceive
two souls that are both different and partless (20:285). Therefore, if
substances are manifold and partless, they must have representational
capacities. The crucial error in this reasoning lay in supposing that
our abstract conceptions are a guide to reality behind the
spatio-temporal appearances. Yet properly understood, he maintained,
Leibniz's monadology was not an attempt to explain appearances
but the expression of a "Platonic" view of the world,
considered apart from our sensory experience of it (4:507; 8:248). In
this regard, he did grasp that Leibniz did not after all take
appearances for things in themselves.



Despite his warnings about the limited powers of human reason, Kant
too believed it possible to deduce some features of matter, as
physical science must theorize it, *a priori*. There are no
material atoms; matter is divisible to infinity and its parts are all
material (4:503f). Yet Kant recognized, first in the *Physical
Monadology*, then in the *Metaphysical Foundations*,
particles in the form of centres of attractive and repulsive forces
that account for the space-filling property and impenetrability of
matter (4: 533ff). This relatively dogmatic treatment co-exists with
his critical claim that matter is the appearance of a perfectly
unknown substratum. As he explains it in the *Critique of Pure
Reason*, the rainbow is a mere appearance relative to rain drops
which, in a physical sense, are things-in-themselves and not mirages.
Yet thinking further, we realize that the raindrops too are mere
appearances, and that "even their round form, indeed, even the
space through which they fall are nothing in themselves, but only mere
modifications or foundations of our sensible intuition; the
transcendental object, however, remains unknown to us." (KRV A
45f/B 63f). "About these appearances, further, much may be said
a priori that concerns their form but nothing whatsoever about the
things in themselves that may ground them." (KRV A49/B66). This
suggests that the stuff which is divisible to infinity and bears
attractive and repulsive forces is an appearance of something unknown
and unknowable. "We can understand nothing except what brings
with it something in intuition corresponding to our words. When we
complain that we do not see into the inner nature of things, this can
mean no more than that we cannot grasp, through pure reason, what the
things that appear to us might be in themselves.... Observation
and division with respect to the appearances take us into the interior
of nature, and we cannot say how far this will proceed. But every
transcendental question that takes us beyond [perceptible] nature can
never be answered..." (KRV (A277f/B333f).


## 5. Space and Time



Leibniz held a relational theory of space and time. Without things
there would be no space, and without events there would be no time.
Space and time are not containers into which things and events may be
inserted but which could have remained empty. In the *Third Letter
to Clarke* (G 7:364), Leibniz maintains that "without the
things placed in it, one point of space does not absolutely differ in
any respect whatsoever from another point of space." An even
more ambitious positive proposal makes space the "order of
co-existents" and time the "order of successions" (G
7: 363), or a "well-founded phenomenon."



Despite his admiration for Newton, his presentation of a purported
proof of absolute space in the essay *On the Basis of the
Difference of Regions in Space* of 1768 (2: 378), and his claim in
the First Critique that two distinct but absolutely identical portions
of space were possible (KRV A 264/ B320), Kant rejected absolute space
and absolute motion in the *Metaphysical Foundations of Natural
Science*. Consistently, however, he rejected Leibniz's claim
that space was founded on the order of relations of substances. To the
claim that space emerged somehow from the underlying monadic reality
seemed to Kant to imply that the truths of mathematics--in this
case three-dimensional geometry--depended on the existence of a
world of things and events, which was absurd. Leibniz, Kant, suggests,
had noticed that things appear to interact causally and to determine
one another's behaviour. This led him to insist that space was
" a certain order in the community of substances, and...
time... the dynamic sequences of their states," confusedly
apprehended (KRV A 275f/B 331f). However, the composition of bodies
from monads as basic elements presupposes their juxtaposition in space
(20:278). If we confusedly perceive monads as physical objects in
space, what would it be like to apprehend the monads distinctly as not
in space but as the foundation of space? (4:481f)



Kant's conviction that the existence of incongruent counterparts
proved that "space in general does not belong to the properties
or relations of things in themselves" (4: 484) is not easy to
understand, but his basic argument in the 1768 essay is that
Leibniz's view does not enable one to distinguish between a left
handed glove and a right handed glove, insofar as the relations of all
the parts to one another are the same in both cases. Yet if God had
created just one glove, it would have been one or the other. Hence
space does not depend on relations between things in space.
Newton's conception of space as a huge container does not
contribute to the solution of the problem: Consider a container in
which a single glove is floating. Is it a right-handed glove or a
left-handed glove? We can insert various new items into this
space-container, e.g., an anorak, a scarf, a shoe, but only the
insertion of a human observer into the space will permit an answer.
Space, Kant, decides, is related to directionality or orientation. The
human observer experiences himself as intersected by three planes and
as having three sets of "sides", which he describes as up
and down, back and forward, and right and left. Right-handedness and
left-handedness are not merely anthropic concepts since nature itself
insists on handedness in twining plants and the shells of snails (2:
380). But which direction is right and which is left can only be
established by a conscious, embodied being. As he expresses it in the
*Prolegomena*, "The difference between similar and equal
things which are not congruent...cannot be made intelligible by
any concept, but only by the relation to the right and left hands,
which immediately refers to intuition" (4: 286). How spheroid
beings with hands would distinguish between "front" and
"back" is not however clear. It is not clear whether this
orientational analysis implies that wherever there is space there must
also be sentient beings with pairs of incongruent parts, as well as
top-bottom and back-front asymmetry.



In the *Inaugural Dissertation* SS15, Kant tries to move
beyond the dichotomy of taking space and time to be either substances
or phenomena, taking space instead to belong to the
"form"of sensible intuition. As he expresses it in the
*Critique of Pure Reason*, "Space and time are [our
sensibility's] pure forms..." (KRV A42/B60). They are
"merely subjective conditions of all our intuition."
(ibid., A 49/B 66). [See the entry on
 Kant's views on space and time.]



For Leibniz, each monad experiences an individual succession of
appetites and experiences that God has endowed it with from the
creation of the world. Being fated in this way, though acting always
under their own "spontaneous" power, the monads would seem
to be deprived of meaningful agency and beneficiaries or victims of
seemingly unjust rewards and punishments. Kant believed that locating
space and time in us rather than in the world was an absolute barrier
against the determinism that threatened the notion of moral
responsibility (A 5:97-8; 102).


## 6. Perception and Thought



Kant's rejection of Leibniz's Principle of the Identity of
Indiscernibles is related to his complaint that the Leibnizians
treated perception and thought as a single representational faculty
that was "logically" (by which Kant meant
"qualitatively") distinguished in terms of the clarity of
the representation, rather than "transcendentally." With
their theory of a single cognitive faculty, Kant maintained, Leibniz
and Wolff "abolished the distinction between phenomena and
noumena to the great detriment of philosophy." Leibniz treated
the senses according to Kant as an inferior mode of cognition, the
senses having only the "despicable task" of confusing and
distorting the representations of reason (KRV B. 332).



The basis of this accusation was Leibniz's attribution to the
soul of only two basic faculties: perception, the representation of
multiplicity in a simple soul, and appetition, which he defines as
"the action of an internal principle which brings about change
or the passage from one perception to another" (G 6:
608-9). Perceptions, in Leibniz, as in Descartes, are thoughts
-- presentations in the mind. The perceptual thought that there
is a green tree in front of me is not absolutely unlike the
mathematical thought that triangles have three angles. Perception of
material objects is "confused" because -- according
to the Cartesian tradition -- corporeal substance has no colours
or other sensory properties -- which come into being through the
interaction of the mind and tiny, single imperceptible, uncoloured
corpuscles of matter. Though Leibniz denied the existence of purely
material corpuscles and the possibility of causal influx or even
interaction between real substances, he agreed that, from the
perspective of what he sometimes terms mere physical science,
perception required interaction, and that corpuscular motions were
involved in the perception of sensory qualities such as light and
colour. Perception can therefore be said to confuse what reason
delivers to us clearly (A 132, 219, 403). Though we are unable to
grasp the sufficient reason of particular colours, there is nothing
arbitrary in their connection with their underlying causes (A
382f).



Leibniz's interpreters, Wolff and Baumgarten, enunciate more
dogmatically the theory that there is a single faculty of
representation in the soul, with perception and cognition
corresponding to its lower and higher parts. It was this view that
Kant presented as entirely opposed to his own teaching with regard to
the faculties of the soul. Sensibility, for Kant, is the
"receptivity of the subject by which it is possible for the
subject's own representative state to be affected in a
particular way by the presence of some object." Thought is
"the faculty of the subject by which it has the power to
represent things which cannot by their own quality come before the
sense of that subject" (2:392). Thought implies an ability to
experience representations of another type not involving the forms of
sensibility -- space, time, and causality. Spatio-temporal
features are attached to all our perceptions and to our perceptual
thoughts, but not to the concepts we entertain descriptively. We can
*think of* things in themselves, and even of God, the soul, and
other such entities, acknowledging their existence and even their
powers, but we do not perceive them and we cannot represent them in
sensuous form.



In his late philosophy, Leibniz distinguished between "simple
monads," which despite their representational and appetitive
faculties, merely experienced something like our swoons and dreamless
sleeps; "souls," belonging to animals with sensory organs
that had awareness of an environment and desires; and
"spirits," who were able to grasp necessary truths and
experience more complex appetitions such as desiring the good (G 6:
610-12). All monads, according to Leibniz, are confusedly
omniscient (G 6: 604). Kant was scornful of what he regarded as
transcendental reverie. The "slumbering monad" with its
dim presentations, is, he complains, "not explained, but
made-up" (2.277). The notion that the slumbering monads might
awake to climb the ladder of awareness recalls, he said, "a kind
of enchanted world" (20:285). However, Kant was favourably
disposed, at least in his early years, towards Leibniz's
doctrine of confused omniscience, and he essentially accepts
Leibniz's conception of the mind as innately stocked with, as
Leibniz expresses it, "inclinations, dispositions, tendencies or
natural potentials" (A 52). In his early essay on *Negative
Quantities*, Kant remarks "There is something great and I
think, very correct in Leibniz's notion that the soul embraces
the entire universe with its representational powers, although only an
infinitely small part of this representation is
clear......Outer things can carry the condition of their
presentation, but not the force to bring themselves into existence for
us. The powers of thought of the soul must have real
grounds..." (2.199). Though Kant later professed
agnosticism as to whether perception and mentality in general were
explicable mechanically, the emphasis on the active powers of the mind
by contrast with the passivity of matter remains important in his
theory of mind.



The distinction between perception and thought signaled Kant's
break with rationalist metaphysics but enabled him to deploy a
divide-and-conquer strategy against dogmatic claims. By showing how
each mode of apprehension involved certain necessary and distinct
limitations on its employment, Kant was able to show that certain
kinds of assertion in theology and metaphysics could not be genuine
knowledge-claims. Perception was limited by the kinds of bodies we had
and the manner in which we could be affected by external objects. We
could not acquire scientific knowledge of the origins of the universe,
or of our condition after death. Pure reason could not fill in details
that were beyond all possible experience. The claims of metaphysics
had to be synthetic, informative and not true by definition, yet *a
priori* true. Arithmetic and geometry supplied synthetic *a
priori* truths in abundance, and natural science supplied
synthetic *a posteriori* truths, as well as exhibiting
synthetic *a priori* propositions, such as the conservation of
force.



Already in his prize essay of 1764, the *Investigation of the
Intelligibility of the Fundamental Principles of Natural Theology and
Morals*, Kant claimed that moral and theological principles were
not capable of demonstration, since their terms, unlike mathematical
terms, lacked precise definition. Geometrical concepts lend themselves
to use in demonstrations because they are constructed and presented to
intuition, which is impossible with respect to metaphysical concepts
such as the soul. In the *Critique of Pure Reason*, he says:
"If anyone were to pose the question to me: What is the
constitution of a thing that thinks? then I do not know the least
thing to answer *a priori*, because the answer ought to be
synthetic...But for every synthetic solution, intuition is
necessary; but this is entirely left out of so universal a
problem" (KRV A 398). The *Prolegomena* returns to the
question how metaphysics can, like natural science and mathematics,
employ synthetic judgments when its concepts are not given in
experience and are not constructed. Kant's answer is that
metaphysical judgments do not point to objects existing beyond all
possible experience, but posit objects needed to
"complete" our understanding, that is, to make our
thinking systematic and untroubled by gaps and aporias. The soul is
not a supersensible object of whose faculties and powers we can
acquire knowledge but an idea that makes our practice of ascribing
experiences to ourselves intelligible.


## 7. Soul and Body



For Leibniz, "I" am a substance, and my mind, as a
"dominant monad," rules over, or expresses more distinctly
than they do,the subordinate monads composing my body. Thus all
happenings in all parts of the body are felt distinctly or
indistinctly, the sensory organs collect and concentrate the
impressions of the external world, and the soul experiences them.
Though commentators disagree over whether the referent of
"I" is a corporeal substance -- a soul-body composite
-- since Leibniz did not believe separated souls without organic
bodies were possible -- or, alternatively, a single dominant
monad, an immaterial substance outside space and time, "I"
assuredly named a thing that was indivisible and imperishable (G 6:
598-600). Kant eschews dogmatism both with respect to whether
the soul is an immaterial substance and whether it is immortal. The
use of the term "I" presupposes that my thoughts and
perceptions are experienced as bound up together and as belonging to a
single entity. Matter, with its properties of extension,
impenetrability, etc., cannot be conceived as producing thought. But
matter is only an appearance; whatever supersensible thing it is that
gives rise to the appearance of matter, that thing may well be the
same as whatever supersensible thing it is that gives rise to the
experience of an experiencing self (KRV A 358f/B 428f).



Leibniz's theory of soul-body pre-established harmony set out in
his *New System of the Nature and the Communication of
Substances* of 1695 is not easy to reconcile with the
interpretation of his monadology according to which what we call
bodies are appearances in visual space founded upon spiritual
substances lying underneath the spatio-temporal order. Yet the
pre-established harmony was at least consistent with Leibniz's
claim that substances do not interact with one another and that what
we call "causal interaction" does not involve a flow of
power or force, but merely a regular sequence of changes in two
observable things, in the case of mind and body, the experiences of
perception and appetition and states of the sensory organs (G
4:76-7). Kant points to the tension between the theory of
pre-established harmony and the monadology; "Why should one
admit bodies, if it is possible that everything happens in the soul as
a result of its own powers, which would run the same course even if
entirely isolated?" (8:249).



Kant initially preferred "influx" theories of soul-body
relations to the "parallelist" theories of occasionalism
and pre-established harmony, but he eventually decided that dualism
was incoherent. Already in the *Measurement of Living Forces*,
he was grappling with the problem of the location of the soul and the
nature of its action. Anatomists had long speculated that some region
of the brain, e.g., the pineal gland (Descartes 1650) or the corpus
callosum (Euler 1763), was the site of interaction between soul and
body. For a time, Kant seems to have believed that souls were
positioned in space and could act outside themselves and be acted upon
by bodies. Later he became convinced that souls were not localized in
space, though they could effect changes, deciding that that neither
medicine nor metaphysics could illuminate the question. He denies that
we can understand the entry of the soul into body at conception or its
relation to the body throughout life, or its exit and separate
existence after death. Because all our experience is experience of
ourselves as living beings -- when soul and body are bound
together -- we cannot know what a separated soul would
experience. Leibniz compared existence after death to a deep sleep or
a swoon, but to investigate these matters is, Kant says, like standing
before a mirror with your eyes closed to see what you look like when
you are asleep (20:309).


## 8. Freedom and Agency



Leibniz believed that every phenomenon could be explained. His
Principle of Sufficient Reason states that "nothing takes place
without a sufficient reason; in other words,...nothing occurs for
which it would be impossible for someone who has enough knowledge of
things to give a reason adequate to determine why the thing is as it
is and not otherwise." Though not everything possible happens
(and, therefore not everything that happens is necessary), everything
that happens has a sufficient reason in an antecedent state of the
world. God's necessary existence is the only state of affairs
that is caused and does not have a sufficient reason in an antecedent
state. Not only does everything have a sufficient reason, but all
phenomena and events, including celestial motions, the formation of
plant and animal bodies, and the processes of life, are regulated by
the laws of mechanics, as the movements of the hands are regulated in
a watch (G 7:417-8).



Leibniz's Principle was incompatible with the existence of an
open future and with free will. His followers recognized this aspect
of his system, though his *Discourse on Metaphysics*, in which
determinism is tied to his inclusion theory of predicate-subject
relations remained unpublished until the twentieth century. Though
Leibniz tried to avoid contradicting the theological dogma of free
will outright, he denied that any creature could choose between
alternatives to which it was indifferent, and he agreed with Locke
that we are powerfully and necessarily motivated by disquiet and
restlessness, which are, in Leibniz's view, sometimes
unconscious or subliminal (A 188f). My body is a machine in a wider
mechanical system, and my thoughts and desires, including my
"petites perceptions," cannot but harmonize with or
parallel states of that machine. "The organized mass in which
the point of view of the soul is located, being expressed more
immediately by it, [is] reciprocally ready to act on her account
following the laws of the corporeal machine, at the moment when the
soul wills it, without one disturbing the laws of other, the animal
spirits and blood taking on, exactly the motions required to
correspond to the passions and the perceptions of the soul" (G
4: 484). Yet, for Leibniz, the infinite complexity and uniqueness of
any living machine make human actions unpredictable, and the truth of
determinism is consistent with our experiences of self-control,
self-management, and behavioural reform (A 195f). For a variety of
reasons, Leibniz did not see determinism or mechanism as a threat to
morality.



Kant did. His perception was facilitated by a series of attacks on
Leibniz's disciple Christian Wolff by theologians alarmed by
what they saw as the horrific consequences of Sufficient Reason,
attacks that led to Wolff's banishment from the University at
Halle. Leibniz's "spiritual automaton," moved by its
presentations, has a freedom, Kant claims in his second critique, the
*Critique of Practical Reason*, that is only
"psychological and comparative." If Leibniz is right, we
have no more than a "freedom of the turnspit" (5: 97),
wound up to run by itself. In that case, Kant thought, man is a
"marionette" (5:101), and morality is but a figment of the
imagination. To know that the moral law is not a figment and is
genuinely binding, it might seem that we would have to know that we
have the power to redirect the forces of nature. Of course we cannot
know this, but, on the other hand, we cannot prove that no such power
exists. [See the entry on
 Kant
 (Sect 5.2) and the entry on
 Kant's moral philosophy
 (Sect. 10).]



Reason presents compelling arguments for the inevitable nature of
every event. Reason also presents compelling arguments that the human
will can influence the course of nature (KRV A 445/B 473). The
antinomy is dissolved, Kant now maintains, by recognizing that causal
relations must structure outward phenomena. Our investigations of
nature presuppose that they do, insofar as they are scientific. Human
agency is not, however, an outward phenomenon, and the assumption of
determinism is not required. We can view ourselves as machines,
responding to the environment in predetermined ways. However, we are
not compelled to do so, and we are able to regard ourselves as agents
who initiate trains of events, and who can resist (not just experience
resistance to) the desires, sensations, and impulses driving the
bodily machine to certain actions. Since we can do so, Kant decides,
we *should*: we need not in that case be driven to or fall into
libertinage on the basis of speculative doctrine. What reason cannot
settle theoretically, she can nevertheless decide on
"practical" grounds, i.e., deciding to believe one thing
rather than another to preserve satisfaction (by contrast with anxiety
and despair), and to give support to our sense that morality is not a
figment. We ought therefore to conceptualize our possession of free
will as an exemption from the laws of nature; the power of
"doing and forbearing." (5: 95).


## 9. Mechanism and the Order of Nature



Kant was perturbed by Hume's criticisms of causal relations in
nature, implying that there were no laws of nature for science to rely
on, and by the antitheological application Hume made of his causal
skepticism in the *Dialogues Concerning Natural Religion*. [See
the entry on
 Kant and Hume on causality.]
 He sought a third way between the Leibnizian "dogmatic"
assumption that the universe is a single mechanical system of
deterministically interacting physical parts, designed and set going
by God, and the empiricist assumption that causality corresponds to a
human feeling of anticipation with respect to some sequence of ideas.
For scientific purposes, Kant thought, we must represent inorganic
nature to ourselves as just such a unified mechanical system. The
resolve to represent it as such is facilitated (or perhaps dictated?)
by our inability to experience a world unstructured by spatial,
temporal, and causal relations. If Leibniz erred by ascribing to
unknown noumena the properties of causal closure belonging to
phenomena, Hume erred by being unaware of the inbuilt constraints on
our representational capacity. But must we represent only inorganic
nature -- stones, stars, nebulae, planets, billiard balls --
as a set of mechanically interacting mechanical systems, or
earth's plants and animals as well? Leibniz was fully committed
to the Cartesian claim that plants and animals are machines, no
different in principle from automata built from wood and metal parts;
though, impressed by the detail revealed by the early microscope, he
described them as infinitely complex machines, "machines in
their smallest parts, into infinity" (G 6: 618), another
indication of the divine origins of nature. Generation and growth were
on his view, mechanical processes, for, according to the doctrine of
preformation, which he shared with Malebranche, generation is just
growth.



Kant was not so sure. By the late 18th century, the theory
of inorganic nature, thanks to Laplace, Black, Priestley, Franklin,
and other chemists and electricians was flourishing, but so too was
the study of physiology, embryology, and natural history, thanks
especially to Bourguet, Boerhaave, Haller, and Buffon. Newtonian
forces acting over a distance were no longer seen as incompatible with
a commitment to mechanism, opening the door to the supposition of
vital forces that might act in a lawlike fashion. Preformation was no
longer a credible doctrine; the possibility of self-assembling
"organic molecules," working according to "organic
mechanism," was much discussed. Epigenesis reduced the need for
a divine creator. Kant addresses the ensuing intellectual-theological
crisis in the *Critique of Judgment*, a two-part essay dealing
with beauty, beautiful forms in nature, and forms in nature generally.
He tries to show that we are caught up in an antinomy. We are strongly
disposed to view visible nature as a unity in which a single set of
mechanical forces operates, not to divide it into an inorganic realm
that came to be by the forces inherent in nature and an organic realm
of plants and animals evidencing design and supernatural creation. Yet
we cannot envision explaining generation or organic growth
mechanistically. The solution to the dilemma is to adopt teleology as
a regulative principle. We should not positively declare that organic
beings could not have arisen and cannot reproduce themselves from the
forces of nature, or that God must have a hand in their genesis;
nevertheless, in investigating them, we look for the function and
interrelation of parts, as though they were designed and built
intelligently (5: 416ff). The claim that the parts of a living
creature are organized *in infinitum* as Leibniz supposed is
however, "something that cannot be thought at all." (KRV A
526/B 554).



Leibniz was often mistakenly credited in the 18th century
with the view that organic nature contained no gaps, i.e., between any
two different-looking organisms, another can be found. Though such a
view might seem consistent with Leibniz's Principle of Plenitude
-- that the universe is as full as possible -- and his Law
of Continuity -- his denial that nature makes leaps (GM 6: 240)
-- it is inconsistent with his view that not everything possible
exists, but only what is compossible with other existents, and Leibniz
held no such view in any case. Kant describes the idea of perfect
continuity in any case as mere intellectual prejudice, since
observation of nature does not objectively support it. However, he
allows the "law of the ladder of continuity among
creatures" has regulative importance in natural history (KRV A
668/ B 696).


## 10. Theology and Theodicy



Leibniz's philosophy is theocratic. God is a king, and the world
is his kingdom. Ours is the best of all possible worlds, with respect
to variety, order, location, place, time, efficiency, and "the
most power, knowledge, the greatest happiness and goodness in created
things" (G 6: 603). For God can choose to realize any world he
wishes, and it would be inconsistent with his goodness and power to
realize a world that is not as good as possible. Our world metes out
justice for all in the hereafter and is constantly improving. The
"kingdom of nature," in which everything happens for
mechanical reasons, is at the same time, a "kingdom of
grace" in which everything that happens exemplifies God's
wisdom and justice (G 6:622). Yet, though Leibniz maintained
frequently that the order and regularity of nature hinted at or
pointed to a divine creative hand, and suggested that the existence of
anything at all implied the existence of a necessary being (G 4: 106),
he produced only one actual argument for the existence of God. This
was a version of Anselm's ontological argument. Leibniz
maintained that God's existence could be deduced from the
maximal concept of God as the sum of all perfections, only if it was
first demonstrated that God was a possible, not an impossible object.
Certain maximal concepts, such as "the greatest speed"
are, he pointed out, fundamentally incoherent, and the term denotes
nothing (G 4: 359-60). Leibniz saw nothing incoherent in the
maximal notion of "the most perfect being" and concluded
that God existed. It was unclear however, why, so long as God is
possible, the Ontological Argument survives the familiar criticisms of
Aquinas. The inference from possibility to necessity seems to depend
less on the logic of concepts than on Leibniz's esoteric idea
that concepts, or possible objects, strive to come into existence with
a tendency to exist proportional to their perfection (G 7:303).



Kant criticized arguments for God's existence (presumably
brought to his attention through Wolff's *Theologia
Naturalis*) that departed from the premise that the concept of God
is noncontradictory as fallacious hyperrationalism. The Leibnizian
argument rested, he thought, on the uncritical notion that every
noncontradictory concept was a possible thing (20:302), though it is
unclear why he attacks this claim in particular. Kant did not think
any rationalistic proofs for the existence of God actually worked,
though, at least before the mauling it received at the hands of Hume,
he considered the physico-theological argument the best available. He
noted perceptively the arbitrariness of merging into one theological
idea a creator and a judge. Without revelation, we might be drawn to
the ideas of a creator God, but why would we suppose this same being
to have the power of reward and punishment after death? Taking a page
from Locke, Kant decided that, since the existence of God was
unknowable, philosophical effort should be directed to the idea of
God, especially the separate functions that the idea of God plays in
regulating our moral conduct ( judge concept) and our mode of address
to the problems of form and function in the organic realm (creator
concept). The distinction between the realm of grace and the realm of
nature, one standing under moral laws of reward and punishment, the
other under natural laws, Kant describes as a "practically
necessary idea of reason" (KRV A 812 f/B 840). We conceive the
world both as a community of active spirits, willing and representing,
and as an aggregate of objects in mechanical interplay and shift
perspectives as needed.



Physical and increasingly moral evil were topics much discussed in the
18th century, and pessimism over the condition of the world,
especially its violence and suffering, was understood to be a tempting
yet in some ways deplorable option.. As befit a celebrated optimist,
Leibniz had a sanguine view of human beings. Most of us, he appeared
to think, are morally decent people, and evil men are best dealt with
by good laws and effective legal institutions. A good education and
some degree of censorship are also helpful. Posthumous divine
retribution will take care of what human institutions cannot. Leibniz
explained the appearance of evil in the world as consistent with
God's goodness in various ways. He claimed that evil derives
from the portion of inertia or nullity present in all non-divine
creation; that it is a necessary accompaniment of good, or a stimulus
to action, that it is an illusion based on narrow or temporally
limited experience (G 4: 120f, 196, 231). Moreover, the world has not
declined since the Fall, but, on the contrary, the entire universe
"takes part in a perpetual and most free progress, so that it is
always advancing toward greater culture (cultivation)" (G 7:
308).



Taken to ridiculous lengths by Wolff, Leibnizian optimism was targeted
and satirized by Voltaire, who, however, took the problem of evil
seriously. Kant as well was little inclined to make fun of evil and
suffering, and he viewed liberation from theology as a precondition of
better morals and politics. There are both optimistic and pessimistic
strains in his philosophy. In his pre-critical writings, the *Nova
Dilucidatio* and the *Essay on Some Considerations on
Optimism*, he seemed inclined towards the best of all possible
worlds position, but in his critical period he denied that cosmic
justice is an item of knowledge. The conviction that moral goodness
not only deserves reward but is rewarded is nevertheless an article of
faith and hope that sustains morality.


## 11. Ethics and Politics



Kant's ethics and political theory overlap with but also depart
from Leibniz's. Leibniz argued that justice requires not only
that one forebear from harming others or giving them cause for
complaint; justice also requires helping them by seeking their good
and preventing evil when doing so is not too difficult. (Riley ed.
53-5). He suggested a variety of social improvements and measures for
the remediation of poverty and anticipated the universalist
formulations of Kant's maxim-test from the *Groundwork*.
"Put yourself in the place of another, and you will have the
true point of view for judging what is just or not." Although it
is 'impossible to act so that the whole world is content,'
the affected ones can be made as content as possible (Riley ed. 56-7).
Kant's ethics by contrast does not make the enhancement of
welfare or the avoidance of harm criteria of moral goodness or
rectitude, though some commentators would insist that his
universalization test implicitly depends on such criteria. He
acknowledges a traditional duty to benevolence but gives no
socio-political examples of welfare policies and shows himself
distinctly sceptical about what he considered exaggerated,
tender-hearted utilitarianism (25:504-5). 



Leibniz considered some forms of slavery just, insofar as some humans
were similar to dumb, ineducable animals, and because slavery is
contingent and temporary because life is eternal (Riley, 78). Kant
insisted on a sharper animal-human divide, was not sure about
immortality, but he considered only the enslavement of criminals just
(6: 332-3). American aboriginals and Africans are also described as
'fit only to be slaves' (XV:181). The inferior races would
die out or be exterminated ( (XV: 878). White, male, Christian
Europeans could, he hoped, in time bring civilization to the entire
world. Leibniz's early views were no more savoury;
'barbarians,' whom he considered 'beasts' (see
Perkins, 111), could be destroyed in order to civilize them.
Leibniz's racism reflects the snobbery of the courtier, which is
at any rate courteous to women, while the Kantian tendency to
pathologize the other flowed rather from an excess of moral fervour
and his vigorous antifeminism. True to his rejection of the Principle
of Contradiction, Kant saw the human soul as the battleground of
animalistic instincts vs. moral duties. Self-indulgence, credulity,
and indolence, which were unfortunately built into the character of
women and nonEuropeans were to be combated by moral rectitude,
education and critique, and industry. 



Yet the deprivations that had to be endured by a person of strong
moral fibre exercising a good will were, Kant recognized, real
deprivations. If the struggle for self-mastery and self-improvement
that he urged on his readers was not to seem, and indeed to be a
pointless exercise, then knowledge, as he expressed it in the Preface
to the second edition of the *Critique of Pure Reason*, had to
be denied in order to make room for faith (B xxx). Alarmed by colonial
depredations, Kant nevertheless saw social and geopolitical conflict
as necessary, at least until his second thoughts in his 1795 essay on
'Perpetual Peace'. Like Leibniz, he tried to find
redeeming aspects to group aggression and interracial conflict as
preconditions of pacification, civilization, and progress. His own
concept of development, of the unfolding of latent potentials, is
central to his anthropology and his philosophy of history. Development
was, however, as much of a duty as an inevitability. He accepted
Leibniz's teleology of history: "We should be content with
providence and with the course of human affairs as a whole, which does
not begin with good and then proceed to evil, but develops gradually
from worse to better..." (8:123). In his 1795 work, he
arrived at the view that warfare was a scourge that would eliminate
itself when exhausted and enlightened people would finally refuse to
finance the rivalries and glory of princes. Leibniz, too, even more
dependent than Kant on the favour of rulers, seems to have abandoned
his earlier militarism in old age (see Wilson). 



For Kant, philosophy is a stern discipline frequently calling for
graceless prose. After publishing several floridly written popular
works, and after acquiring his knowledge of physics through such
accessible expositions as Bernard Fontenelle's *Dialogues on
the Plurality of Worlds* (1696), Emilie du Chatelet's
*Institutions of Physics* (1740), and Euler's *Letters
to a German Princess* (1768-72) (7:229-30), Kant
traded literary charm for technicality, rigor, and repetition,
especially in the first two *Critiques*. The enchanting views
of the *Monadology* which Leibniz himself may have regarded as
a jeu d'esprit were antithetical to philosophy's true
purpose. Yet Kant finally aestheticizes Leibniz, claiming that he
meant only to express a view of the world that is true in its own way.
The history of philosophy, Kant suggests, is not to be assessed in
terms of right and wrong doctrines. To accuse Leibniz or Plato of
errors is to take them as authorities, as one takes Cicero as the
standard for Latin, and this is a confusion; for "there are no
classical authors in Philosophy" (8: 218).