## 1. Matter and Thought



For present purposes, we may think of materialism as the view that
everything that exists is material (or physical) with this view
closely allied to another, namely, that mental states and processes
are either identical to, or realized by, physical states and
processes. Leibniz remained opposed to materialism throughout his
career, particularly as it figured in the writings of Epicurus and
Hobbes. The realms of the mental and the physical, for Leibniz, form
two distinct realms--but not in a way conducive to dualism or the
view that there exists both thinking substance and extended substance.
By opposing both materialism and dualism, Leibniz carved himself an
interesting place in the history of views concerning the relationship
between thought and matter.



Most of Leibniz's arguments against materialism are directly
aimed at the thesis that perception and consciousness can be given
mechanical (i.e. physical) explanations. His position is that
perception and consciousness cannot *possibly* be explained
mechanically, and, hence, could not be physical processes. His most
famous argument against the possibility of materialism is found in
section 17 of the *Monadology* (1714):



> 
> One is obliged to admit that *perception* and what depends upon
> it is *inexplicable on mechanical principles*, that is, by
> figures and motions. In imagining that there is a machine whose
> construction would enable it to think, to sense, and to have
> perception, one could conceive it enlarged while retaining the same
> proportions, so that one could enter into it, just like into a
> windmill. Supposing this, one should, when visiting within it, find
> only parts pushing one another, and never anything by which to explain
> a perception. Thus it is in the simple substance, and not in the
> composite or in the machine, that one must look for perception. (G VI,
> 609/A&G 215)
> 



Leibniz's argument seems to be this: the visitor of the machine,
upon entering it, would observe nothing but the properties of the
parts and the relations they bear to one another. But no explanation
of perception or consciousness can possibly be deduced from this
conglomerate. No matter how complex the inner workings of this
machine, nothing about them reveals that what is being observed are
the inner workings of a conscious being. Hence, materialism must be
false, for there is no possible way that the purely mechanical
principles of materialism can account for the phenomena of
consciousness. (For detailed discussion of this argument, see Lodge
and Bobro 1998; Lodge 2014.)



In other writings, Leibniz suggests exactly what characteristic it is
of perception and consciousness for which the mechanical principles of
materialism cannot account. The following passages, the first from the
*New System of Nature* (1695), the second from the *Reply to
Bayle* (1702), are revealing in this regard:



> 
> Furthermore, by means of the soul or form, there is a true unity which
> corresponds to what is called the *I* in us; such a thing could
> not occur in artificial machines, nor in the simple mass of matter,
> however organized it may be. (G IV, 482/W&F 16)
> 
> 
> But in addition to the general principles which establish the monads
> of which compound things are merely the results, internal experience
> refutes the Epicurean [i.e. materialist] doctrine. This experience is
> the consciousness which is in us of this *I* which apperceives
> things which occur in the body. This perception cannot be explained by
> figures and movements. (G IV, 559f./W&F 113)
> 
> 
> 



Leibniz's point is that whatever is the subject of perception
and consciousness must be truly one, a single "I" properly
regarded as *one* conscious being. An aggregate of matter is
not truly one and so cannot be regarded as a single *I* capable
of being the subject of a unified mental life. This interpretation
fits nicely with Lebniz's oft-repeated definition of perception
as "the representation in the simple of the compound, or of that
which is outside" (*Principles of Nature and Grace,*
sec.2 (1714)). More explicitly, in a letter to Antoine Arnauld of 9
October 1687, Leibniz wrote that "in natural perception and
sensation, it is enough for what is divisible and material and
dispersed into many entities to be expressed or represented in a
single indivisible entity or in a substance which is endowed with
genuine unity." If perception (and hence, consciousness)
essentially involves a representation of a variety of content in a
simple, indivisible "I," then we may construct
Leibniz's argument against materialism as follows: Materialism
holds that matter can explain (is identical with, can give rise to)
perception. A perception is a state whereby a variety of content is
represented in a true unity. Thus, whatever is not a true unity cannot
give rise to perception. Whatever is divisible is not a true unity.
Matter is infinitely divisible. Hence, matter cannot form a true
unity. Hence, matter cannot explain (be identical with, give rise to)
perception. If matter cannot explain (be identical to, give rise to)
perception, then materialism is false. Hence, materialism is
false.



Leibniz rejected materialism on the grounds that it could not, in
principle, ever capture the "true unity" of perceptual
consciousness, that characteristic of the self which can
simultaneously unify a manifoldness of perceptual content. If this is
Leibniz's argument, it is of some historical interest that it
bears striking resemblances to contemporary objections to certain
materialist theories of mind. Many contemporary philosophers have
objected to some versions of materialism on the basis of thought
experiments similar to Leibniz's, experiments designed to show
that qualia and consciousness are bound to elude certain materialist
conceptions of the mind (cf. Searle 1980; Nagel 1974; McGinn 1989;
Jackson 1982). It is also of historical interest that Leibniz coupled
his rejection of materialism with a version of naturalism (or what
*he* would have regarded as naturalism - see Jorgensen 2019).



Leibniz's rejection of materialist conceptions of the mind was
coupled with a strong opposition to dualistic views concerning the
relationship between mind and body, particularly the substance dualism
that figured in the philosophy of Descartes and his followers.
According to this dualism, the world fundamentally consists of two
disparate substances: extended material substance (body) and
unextended thinking substance (mind). This bifurcation, of course,
carries no burden of holding that the operations of the mental are
realized by the operations of the physical. But despite his claim that
consciousness and perception cannot be realized by, nor reduced to,
the mechanical operations of matter, Leibniz found the alternative of
postulating two distinct kinds of substance equally implausible.



Leibniz's opposition to Cartesian dualism stems not from a
rejection of unextended substance, but from his denial of the
existence of genuine extended material substance. To begin with,
Leibniz held the Scholastic thesis that "being" and
"one" are equivalent. He writes to Arnauld: "To be
brief, I hold as axiomatic the identical proposition which varies only
in emphasis: that what is not truly *one* being is not truly
one *being* either" (30 April 1687; G II, 97/LA 121). For
Leibniz, in order for something to count as a real being--a
substance--it must be "truly one," an entity endowed
with genuine unity. And, as we saw above, in order for something to be
a genuine unity, it must be a simple, indivisible entity.
"Substantial unity," he writes, "requires a
complete, indivisible and naturally indestructible entity" (to
Arnauld, 28 November 1686; G II, 76/LA 94). But matter is extended,
and thus, Leibniz believes, infinitely divisible. Hence, there is no
such thing, for Leibniz, as material substance.



There is a positive thesis which goes hand-in-hand with
Leibniz's negative thesis against material substance and which
helps to explain further his rejection of material substance. It is
summarized in the following passage from a letter to Arnauld of 30
April 1687:



> 
> I believe that *where there are only beings through aggregation,
> there will not even be real beings*. For every being through
> aggregation presupposes beings endowed with a true unity, because it
> obtains its reality from nowhere but that of its constituents, so that
> it will have no reality at all if each constituent being is still an
> entity through aggregation; or else, one must yet seek another basis
> to its reality, which in this way, if one must constantly go on
> searching, can never be found.... If there are aggregates of
> substances, there must also be genuine substances from which all the
> aggregates result. One must therefore necessarily arrive either at
> mathematical points from which certain authors make up extension, or
> at Epicurus' and M. Cordemoy's atoms (which you, like me,
> dismiss), or else one must acknowledge that no reality can be found in
> bodies, or finally one must recognize certain substances in them that
> possess a true unity. (G II, 96/LA 120)
> 



According to Leibniz, bodies (qua material) are aggregates, and an
aggregate, of course, is not a substance on account of its lack of
unity. The claim in the above passage is that whatever being or
reality an aggregate has derives from the being and reality of its
constituents. Thus, Leibniz thinks that if a body is to have any
reality at all, if it is to be more than a mere "phenomenon,
lacking all reality as would a coherent dream," then it must
ultimately be composed of things which are real beings. Atoms, he
claims, are unfit for this role, because they are themselves extended
beings, and for Leibniz, divisibility is of the essence of extension.
That is, those who believe in indivisible atoms make matter
"divisible in one place, indivisible in another" (*On
Nature Itself* (1698); G IV, 513/A&G 165), but "we
cannot explain why bodies of a definite smallness [i.e. atoms] should
not be further divisible" (*Primary Truths* (1686); C
522/A&G 34). Every extended mass, for Leibniz, is composed of
extended parts, and so even if we could conceive of an atom as
composed of parts which cannot be physically divided, "an
invincible attachment of one part to another would not at all destroy
the diversity of these parts" (*New System of Nature,*
(1695); G IV, 482/W&F 16), or it would not at all overcome the
fact that it is an aggregate composed of parts and not truly one
being. Likewise, mathematical points, "even an infinity of
points gathered into one, will not make extension," (to Des
Bosses, 30 April 1709; G II, 370/L 597) and so cannot be understood as
the constituents of extended bodies. Hence, Leibniz opts for the last
of the above quoted alternatives: the constituents of bodies are
"certain substances ... that possess a true unity."
These substances are partless, unextended, and indivisible, and
therefore real beings in Leibniz's sense. Indeed, in several
writings, Leibniz invites us to conceive of these substances on the
model of our notion of souls. These simple substances are the only
things which suffice for grounding the reality of bodies. To be sure,
substances, Leibniz tells us, do not constitute a body as parts of the
body, but as the "first elements," or "primitive
unities," of the body. As he sometimes puts it, bodies
"result from" these constitutive unities. That is, bodies
just are aggregates of substances which *appear to us* as
extended corporeal phenomena, though they are
"well-founded" phenomena; they have their foundation in
real beings.



In short, Leibniz stands in a special position with respect to the
history of views concerning thought and its relationship to matter. He
rejects the materialist position that thought and consciousness can be
captured by purely mechanical principles. But he also rejects the
dualist position that the universe must therefore be bifurcated into
two different kinds of substance, thinking substance and material
substance. Rather, it is his view that the world consists solely of
one *type* of substance, though there are infinitely many
substances of that type. These substances are partless, unextended
entities, some of which are endowed with thought and consciousness,
and others of which found the phenomenality of the corporeal world.
The sum of these views secures Leibniz a distinctive position in the
history of the philosophy of mind.


## 2. Denial of Mind-Body Interaction, Assertion of Pre-established Harmony



A central philosophical issue of the seventeenth century concerned the
apparent causal relations which hold between the mind and the body. In
most seventeenth-century settings, this issue was discussed within the
context of substance dualism, the view that mind and body are
different kinds of substance. As noted above, Leibniz remained
fundamentally opposed to dualism. But although Leibniz held that there
is only one type of substance in the world, and thus that mind and
body are ultimately composed of the same kind of substance (a version
of monism), he also held that mind and body are metaphysically
distinct. There are a variety of interpretations of what this
metaphysical distinctness consists in for Leibniz, but on any
plausible interpretation it is safe to assume (as Leibniz seems to
have done) that for any person *P*, *P*'s mind is
a distinct substance (a soul) from *P*'s body. With this
assumption in hand, we may formulate the central issue in the form of
a question: how is it that certain mental states and events are
coordinated with certain bodily states and events, and vice-versa?
There were various attempts to answer this question in Leibniz's
time period. For Descartes, the answer was mind-body interactionism:
the mind can causally influence the body and (most commentators have
held) vice-versa. For Malebranche, the answer was that neither created
minds nor bodies can enter into causal relations because God is the
only causally efficient being in the universe. God causes certain
bodily states and events on the occasion of certain mental states and
events, and vice-versa. Leibniz found Descartes' answer
unintelligible (cf. *Theodicy,* sec. 60; G VI, 135f.) and
Malebranche's excessive because miraculous (cf. Letter to
Arnauld, 14 July 1686; G II, 57/LA 65).



Leibniz's account of mind-body causation was in terms of his
famous doctrine of the *pre-established harmony*. According to
the latter, (1) no state of a created substance has as a real cause
some state of another created substance (i.e. a denial of
inter-substantial causality); (2) every non-initial, non-miraculous,
state of a created substance has as a real cause some previous state
of that very substance (i.e. an affirmation of intra-substantial
causality); and (3) each created substance is programmed at creation
such that all its natural states and actions are carried out in
conformity with all the natural states and actions of every other
created substance.



Formulating (1) through (3) in the language of minds and bodies,
Leibniz held that no mental state has as a real cause some state of
another created mind or body, and no bodily state has as a real cause
some state of another created mind or body. Further, every
non-initial, non-miraculous, mental state of a substance has as a real
cause some previous state of that mind, and every non-initial,
non-miraculous, bodily state has as a real cause some previous state
of that body. Finally, created minds and bodies are programmed at
creation such that all their natural states and actions are carried
out in mutual coordination.



According to Leibniz, what *appear* to be real causal relations
between mind and body are, in metaphysical reality, the mutual
conformity or coordination of mind and body--in accordance with
(3)--with no interaction or divine intervention involved. For
example, suppose that Smith is pricked with a pin (call this bodily
state *Sb*) and pain ensues (call this mental state
*Sm*), a case of apparent body to mind causation. Leibniz would
say that in such a case some state of Smith's mind (soul) prior
to *Sm* was the real cause of *Sm* and *Sb* was
not a causal factor in the obtaining of *Sm*. Suppose now that
Smith has a desire to raise his arm (call this mental state
*Sm*), and the raising of his arm ensues (call this bodily
state *Sb*), a case of apparent mind to body causation. Leibniz
would say that in such a case some state of Smith's body prior
to *Sb* was the real cause of *Sb* and *Sm* was
not a causal factor in the obtaining of *Sb*. So although
substances do not causally interact, their states accommodate one
another *as if* there were causal interaction among
substances.



It should be noted, however, that Leibniz did think that there was a
sense in which one could say that mental events influence bodily
events and vice-versa. He wrote to Antoine Arnauld that although
"one particular substance has no physical influence on another
... nevertheless, one is quite right to say that my will is the
cause of this movement of my arm ...; for the one expresses
distinctly what the other expresses more confusedly, and one must
ascribe the action to the substance whose expression is more
distinct" (28 November 1686 (draft); G II, 71/LA 87). In this
passage, Leibniz sets forth what he takes the metaphysical reality of
apparent inter-substantial causation to amount to. We begin with the
thesis that every created substance perceives the entire universe,
though only a portion of it is perceived distinctly, most of it being
perceived unconsciously, and, hence, confusedly. Now consider two
created substances, *x* and *y* (*x* not
identical to *y*), where some state of *x* is said to be
the cause of some state of *y*. Leibniz's analysis is
this: when the causal state of affairs occurred, the relevant
perceptions of substance *x* became more distinct, while the
relevant perceptions of substance *y* became more confused.
Insofar as the relevant perceptions of *x* become increasingly
distinct, it is "causally" active; insofar as the relevant
perceptions of substance *y* become increasingly confused, it
is passive. In general, causation is to be understood as an increase
in distinctness on the part of the causally active substance and an
increase in confusedness on the part of the passively affected
substance. Again, each substance is programmed at creation to be
active/passive at the relevant moment with no occurrence of real
substantial interaction.



It is difficult to say exactly why Leibniz denied inter-substantial
causation. Some of the things he tells us, in both private and public
writings, seem unsatisfactory. For example, in *Primary Truths*
(1686?), we are given this:



> 
> Strictly speaking, one can say that *no created substance exerts a
> metaphysical action or influx on any other thing*. For, not to
> mention the fact that one cannot explain how something can pass from
> one thing into the substance of another, we have already shown that
> from the notion of each and every thing follows all of its future
> states. What we call causes are only concurrent requisites, in
> metaphysical rigor. (C 521/A&G 33)
> 



Leibniz's first reason for denying inter-substantial causation,
that "one cannot explain how something can pass from one thing
into the substance of another," is a clear reference to the
*influx theory* of causation. Leibniz found this theory
inconsistent with his conception of substance. Influx theory could
only explain causal relations between entities with parts, according
to Leibniz. It is through these parts that "something can pass
from one thing into ... another." But Leibnizian substances
do not have parts in the requisite sense. (For more on influx theory,
see O'Neil 1993.) Leibniz also gives a reason tied to his
complete concept theory of substance, according to which "the
nature of an individual substance or of a complete being is to have a
notion so complete that it is sufficient to contain and to allow us to
deduce from it all the predicates of the subject to which this notion
is attributed" (*Discourse on Metaphysics*, sec. 8; (G
IV, 433/A&G 41)). But there are, it seems, at least two problems
with this explanation. First, Leibniz moves rather quickly from a
conceptual explanation of substance in terms of the complete concept
theory to the conclusion that this consideration is sufficient to
explain the activity of concrete substances. Second, even if
conceptual considerations about substances were sufficient to explain
their apparent causal activity, it does not seem to follow that
substances do not interact--unless one is assuming that causal
overdetermination is not a genuine possibility. Leibniz seems to be
assuming just that but without argument. 



Sometimes Leibniz gives a more familiar line of reasoning. At
*Monadology* 7, we read this:



> 
> There is no way of explaining how a monad can be altered or changed
> internally by some other creature, since one cannot transpose anything
> in it, nor can one conceive of any internal motion that can be
> excited, directed, augmented, or diminished within it, as can be done
> in composites, where there can be change among the parts. The monads
> have no windows through which something can enter or leave. Accidents
> cannot be detached, nor can they go about outside of substances, as
> the sensible species of the Scholastics once did. Thus, neither
> substance nor accident can enter a monad from without. (G VI,
> 607/A&G 213f.)
> 



He seems to think that causal interaction between two beings requires
the transmission or transposition of the parts of those beings. But
substances are simple unextended entities which contain no parts.
Thus, there is no way to explain how one substance could influence
another. Unfortunately, however, this line of reasoning would seem to
also rule out one case of inter-substantial causation which Leibniz
allows, viz., God's concurrent causal action on finite simple
substances.


## 3. Language and Mind



Some scholars have suggested that Leibniz should be regarded as one of
the first thinkers to envision something like the idea of artificial
intelligence (cf. Churchland 1984; Pratt 1987). Whether or not he
should be regarded as such, it is clear that Leibniz, like
contemporary cognitive scientists, saw an intimate connection between
the form and content of language, and the operations of the mind.
Indeed, according to his own testimony in the *New Essays*, he
"really believe[s] that languages are the best mirror of the
human mind, and that a precise analysis of the signification of words
would tell us more than anything else about the operations of the
understanding" (bk.III, ch.7, sec.6 (RB, 333)). This view of
Leibniz's led him to formulate a plan for a "universal
language," an artificial language composed of symbols, which
would stand for concepts or ideas, and logical rules for their valid
manipulation. He believed that such a language would perfectly mirror
the processes of intelligible human reasoning. It is this plan that
has led some to believe that Leibniz came close to anticipating
artificial intelligence. At any rate, Leibniz's writings about
this project (which, it should be noted, he never got the chance to
actualize) reveal significant insights into his understanding of the
nature of human reasoning. This understanding, it turns out, is not
that different from contemporary conceptions of the mind, as many of
his discussions bear considerable relevance to discussions in the
cognitive sciences.



According to Leibniz, natural language, despite its powerful resources
for communication, often makes reasoning obscure since it is an
imperfect mirror of intelligible thoughts. As a result, it is often
difficult to reason with the apparatus of natural language,
"since it is full of innumerable equivocations" (*On
the Universal Science: Characteristic* (undated); G VII, 205/S
18). Perhaps this is because of his view that the terms of natural
language stand for complex, or derivative, concepts--concepts
which are composed of, and reducible to, simpler concepts. With this
"combinatorial" view of concepts in hand, Leibniz notices
"that all human ideas can be resolved into a few as their
primitives" (*On the Universal Science: Characteristic*;
G VII, 205/S 18). We could then assign symbols, or
"characters," to these primitive concepts from which we
could form characters for derivative concepts by means of combinations
of the symbols. As a result, Leibniz tells us, "it would be
possible to find correct definitions and values and, hence, also the
properties which are demonstrably implied in the definitions"
(*On the Universal Science: Characteristic*; G VII, 205/S 19).
The totality of these symbols would form a "universal
characteristic," an ideal language in which all human concepts
would be perfectly represented, and their constitutive nature
perfectly transparent. Now it is true that Leibniz eventually came to
doubt "whether any concept of this [primitive] kind appears
distinctly to men, namely, in such a way that they know they have
it" (*An Introduction to a Secret Encyclopedia* (1679?);
C 513/MP 7). But it is also clear that he did not see this skepticism
concerning our ability to reach the primitive concepts as much of a
barrier to the project of a universal language. He writes in *The
Art of Discovery* (1685) that "there are certain primitive
terms which can be posited, if not absolutely, at least relatively to
us" (C 176/W 51). The suggestion seems to be that even if we
cannot provide a catalog of absolutely primitive concepts, we can
nevertheless construct a characteristic based on concepts which cannot
be further resolved *by humans*.



In addition to the resolution of concepts, and their symbolic
assignments, Leibniz envisages the formulation of logical rules for
the universal characteristic. He claims that "it is plain that
men make use in reasoning of several axioms which are not yet quite
certain" (*The Method of Certitude and the Art of
Discovery* (undated); G VII, 183/W 49). Yet with the explicit
formulation of these rules for the logical manipulation of the
symbols--rules which humans use in reasoning--we would be in
possession of a universal language which would mirror the relations
between the concepts used in human reasoning. Indeed, the universal
characteristic was intended by Leibniz as an instrument for the
effective calculation of truths. Like formal logic systems, it would
be a language capable of representing valid reasoning patterns by
means of the use of symbols. Unlike formal logic systems, however, the
universal language would also express the content of human reasoning
in addition to its formal structure. In Leibniz's mind,
"this language will be the greatest instrument of reason,"
for "when there are disputes among persons, we can simply say:
Let us calculate, without further ado, and see who is right"
(*The Art of Discovery* (1685); C 176/W 51).



Judging from Leibniz's plans for a universal language, it is
clear that Leibniz had a specific view about the nature of human
cognitive processes, particularly about the nature of human reasoning.
According to this view, cognition is essentially symbolic: it takes
place in a system of representations which possesses language-like
structure. Indeed, it was Leibniz's view that "all human
reasoning uses certain signs or characters," (*On the
Universal Science: Characteristic*; G VII, 204/S 17) and "if
there were no characters, we could neither think of anything
distinctly nor reason about it" (*Dialogue* (1677); G
VII, 191/A&G 271)). Add to this conception Leibniz's view
that human cognitive processes follow determinable axioms of logic,
and the picture that emerges is one according to which the mind
operates, at least when it comes to intelligible reasoning, by
following implicit algorithmic procedures. Regardless of whether or
not Leibniz should be seen as the grandfather of artificial
intelligence, he did conceive of human cognition in essentially
computational terms. In fact, as early as 1666, remarking favorably on
Hobbes' writings, Leibniz wrote: "Thomas Hobbes,
everywhere a profound examiner of principles, rightly stated that
everything done by our mind is a *computation*" (*On
the Art of Combinations* (1666); G IV, 64/P 3).


## 4. Perception and Appetition



What do we find in the human mind? Representations on the one hand,
and tendencies, inclinations, or strivings on the other, according to
Leibniz. Or, to put this in Leibniz's more customary
terminology, what is found within us is perception and appetition.
Human minds count for Leibniz as simple substances, and, as he says in
a letter to De Volder of 30 June 1704, "it may be said that
there is nothing in the world except simple substances, and, in them,
perception and appetite" (G II, 270/A&G 181).



Perception has already been discussed briefly above. But it will be
advisable to consider also a definition from a letter to Des Bosses
(and echoed in many other passages) in which Leibniz discusses
perception as the representation or "expression" of
"the many in the one" (letter to Des Bosses, 11 July 1706;
G II, 311). We shall return to this definition below. Appetitions are
explained as "tendencies from one perception to another"
(*Principles of Nature and Grace,* sec.2 (1714); G VI,
598/A&G 207). Thus, we represent the world in our perceptions, and
these representations are linked with an internal principle of
activity and change (*Monadology*, sec.15 (1714); G VI,
609/A&G 215) which, in its expression in appetitions, urges us
ever onward in the constantly changing flow of mental life. More
technically explained, the principle of action, that is, the primitive
force which is our essence, expresses itself in momentary derivative
forces involving two aspects: on the one hand, there is a
representative aspect (perception), by which the many without are
expressed within the one, the simple substance; on the other, there is
a dynamical aspect, a tendency or striving towards new perceptions,
which inclines us to change our representative state, to move towards
new perceptions. (For more details, see Carlin 2004 and Jorati
2017.)



It should not be inferred that this appetitive tendency to change is
entirely governed by efficient causation only. For in Leibniz's
view, value and final causes are not excluded from the action of the
mind, the change of mental states. As he says in section 13 of the
*Discourse on Metaphysics* (1686), just as "God will
always do the best, ... a man shall always do ... that which
appears to him to be the best" (G IV, 438/A&G 46).
Appearance, of course, has to do with perception; doing, with
appetition. So this principle of human action applies directly, as one
would expect, to the two key factors of monadic interior life, only
with the role of value, or an end in view, now more clearly in focus.
This is why Leibniz says that, at the level of bodies (that is, for
Leibniz, at the level of well-founded phenomena), all occurs according
to the laws of efficient causes; whereas with respect to perceptions
and appetites (or at least with some of these--interpretations
differ here) all occurs according to the laws of final causes. And
although both types of causation can be found at both levels (cf.
Carlin 2006), there is no clash here given the harmony of final and
efficient causes in Leibniz's system.



To be sure, at an ultimate level, the only actions of substances
*are* changes of perceptions. Thus, at the ultimate level, the
appetitions are not so much the tendencies impelling a person towards
voluntary motions of the human body (although at the level of
well-founded phenomena this may indeed be the case) but rather
tendencies arising out of present perceptions (present appearances)
towards new perceptions. This explains why Leibniz defines appetitions
in the initially surprising way noted above, as "tendencies from
one perception to *another*"--another
*perception*, that is.



The last two paragraphs have helped to clarify appetition. It is time
to return to perception. In Leibniz's definition (the expression
of the many in the one) the two key terms are "expression"
and "one." Both of them bear considerable weight in
Leibniz's metaphysics. Representation or expression (Leibniz
uses the two terms interchangeably) has its own definition: "One
thing expresses another ... when there is a constant and
regulated relation between what can be said of the one and of the
other" (letter to Arnauld, 9 October 1687; G II, 112/LA 144).
Examples, in addition to perception, include a map expressing or
representing a geographical region and an algebraic equation
representing or expressing a geometric figure, such as a circle or an
ellipse.



As we saw in the first section, with respect to oneness, Leibniz
famously claims that only something with true unity is truly real. For
Leibniz, what truly *is* is substance, so it is not surprising
that at one point he clarifies his definition of perception by saying
that perception is "the expression of many things in one, *or
in simple substance*" (*A New Method of Learning and
Teaching Jurisprudence*, revision notes of 1697-1700; A VI,
i, 272/L 91).



Finally, it should be recalled that for Leibniz there are quite
distinct levels of perception among created substances. Some of these
will be taken up in more detail in the following section, but the
basic point for now is that the three major levels, from the lowest to
the highest, are bare perception (without special distinctness or
memory), sensation (with heightened distinctness and memory), and
thought (with distinctness, memory, and reflection). These are
distinctive of the three levels of monads, respectively, the bare
monads, souls, and spirits. Only the last of these may properly be
said to have reason. Only the last of these is strictly a mind in the
Leibnizian classification.


## 5. Apperception, Desire and the Unconscious



One of the better-known terms of Leibniz's philosophy, and of
his philosophy of mind, is "apperception." A famous
definition is presented in section 4 of the *Principles of Nature
and of Grace* (1714), where Leibniz says that apperception is
"*consciousness*, or the reflective knowledge of this
internal state." He adds that this is "something not given
to all souls, nor at all times to a given soul" (G VI,
600/A&G 208).



Despite being well known, Leibniz's concept of apperception is
not necessarily well understood. In particular, the place of
apperception in the three-fold classifications given at the end of the
previous section--of three kinds of perceptions and of simple
substances--is not agreed upon, despite the fact that this would
seem to be of considerable importance. (See Kulstad 1991a for
discussion.) A common understanding is that for Leibniz apperception
is distinctive of spirits and is not present in even the highest of
animals beneath humans. While there is evidence that Leibniz at least
sometimes adopts this position, there is also evidence that he
sometimes endorses the view that (at least some) beasts also
apperceive. Since we may assume that at a minimum apperception
involves consciousness (though not necessarily certain higher forms of
consciousness, e.g., self-consciousness, or reflective consciousness,
in one sense or another), this leads to some uncertainty as to whether
Leibniz assigns consciousness to beasts, that is, whether he does or
does not agree with the famous Cartesian principle that beasts are not
conscious but only material automata.



There are at least three specific lines of evidence for apperception
in beasts. The first is that Leibniz sometimes uses very similar
definitions and examples when talking about the contrast between, on
the one hand, apperceptions and *petites* perceptions
(perceptions which are not apperceived), and, on the other, sensation
and bare perceptions. This suggests, though it does not demonstrate,
that Leibniz is identifying apperception and sensation, not
apperception and rational thought. The second line of evidence is that
he often appears to take the side of the common man against
Descartes' position on beasts, for example, when he says,



> 
> It will be difficult to rid mankind of this opinion which has been
> held always and everywhere and which is universal if any opinion
> deserves that term, namely, that beasts have feelings (letter to
> Arnauld, 9 October 1687; G II, 117/LA 150).
> 



Finally, there are passages, notably in the *New Essays concerning
Human Understanding* (1704), in which Leibniz quite simply
ascribes apperception, directly or indirectly, to beasts, as, for
example, when he discusses the case of a wild boar that has only a
bare perception of a human until the human shouts at it, at which
point the boar apperceives the person
("*s'appercoit d'une personne*")
and begins a charge (Bk.II ch.21, sec.5; A vi VI 173). 



Without trying to proceed further with this issue here, we can see
that whichever of these views is ultimately adopted, it remains the
case that Leibniz's theory of perception involves something very
distinctive in an age dominated by Descartes' theory of ideas,
the thesis that there are some perceptions of which we are not
conscious, the much-discussed *petites* perceptions. Although
Leibniz may not have been the first to propose such an idea (Aquinas,
for example, might have had a similar view; see his *Summa
Theologiae*, I-II, q.1, art.3), and although the view in his hands
did not have the explosive quality that it did in the hands of Freud,
the thesis remains an intriguing and important part of his philosophy
of mind. Indeed, the Preface of the *New Essays concerning Human
Understanding* contains as strong a statement as one is likely to
find about the centrality of this view in a particular metaphysical
system. Among other things, Leibniz makes it very clear that it is not
just lower simple substances that have such unconscious perceptions
but also human minds.



Having raised the issue of unconscious perceptions, we should consider
also the question of unconscious appetitions. This is infrequently
discussed, but the question should not be overlooked. Since
appetitions are tendencies or strivings, ones which profoundly
influence human actions, it is relevant whether or not an individual
human is conscious of all of these strivings. Certainly, some have
taken the possibility of urges of which we are not conscious as highly
important for the proper understanding of individual humans and,
indeed, of the human condition generally.



There is evidence, notably from the *New Essays*, that Leibniz
did indeed draw a parallel between perceptions and appetitions with
respect to consciousness. Although he did not always explain the
distinction between conscious and unconscious appetitions with care
and uniformity, it seems clear that he committed himself to
appetitions of which we are not conscious, or which we do not
apperceive, just as he had committed himself to perceptions which are
not apperceived. Consider the following two statements in combination:
"desires and tendencies which are apperceived are often called
volitions" (*New Essays*, Bk.II, ch.21, sec.39; RB 192);
and, "There are also efforts that result from insensible
perceptions which one does not apperceive, and these I prefer to call
*appetitions* rather than volitions (although there are also
apperceptible appetitions)" (*New Essays*, Bk.2, ch.21,
sec.5; RB 173).



In short, and perhaps oversimplifying to a certain extent, we can say
that in the Leibnizian realm of mind there are indeed only perceptions
and appetitions, but in these there is a fundamental divide between
the realm of consciousness and unconsciousness. In the former, there
are apperceptions and desires, the perceptions and appetitions of
which we are conscious. In the latter, there are perceptions and
appetitions of which we are not conscious. That does not mean,
however, that this latter realm is unimportant in our mental lives. As
Leibniz says, "insensible perceptions are as important to [the
science of minds, souls, and soul-like substances] as insensible
corpuscles are to natural science, and it is just as unreasonable to
reject the one as the other on the pretext that they are beyond the
reach of our senses" (*New Essays*, Preface; RB 56). He
would have said the same, no doubt, about inapperceptible
appetitions.