## 1. Life and Works




Regius was born in Utrecht, The Netherlands (the Republic of the Seven
United Netherlands at the time), on 29 July 1598, into a wealthy
family of brewers, and died on 19 February 1679 in the same
city. Following initial studies in Arts at the University of Franeker,
he studied medicine at Groningen and Leiden, and subsequently at the
University of Montpellier (France). In 1623 he received his MD from
the University of Padua (Italy). Regius was appointed town physician
in Utrecht on his return there in 1625 and, following a period as
headmaster of the Latin School at Naarden (where he was challenged
about his religious orthodoxy as a Calvinist), he returned to Utrecht
in 1634 and was re-appointed town physician in 1637. The Illustrious
School of Utrecht had become a university in 1636 and, two years
later, Regius was appointed 'extraordinary' Professor of
Theoretical Medicine and Botany (equivalent to an associate professor,
who was paid a significantly reduced salary). The following year,
1639, he was promoted to full professor. In 1640 Regius was awarded a
salary increase, on the understanding that he would provide extra
lectures to his students on natural philosophy. Since there were
already professors of philosophy, mathematics, and theology in place
at Utrecht, all of whom were senior to Regius, this extension of his
contract included the seeds of subsequent controversies about
philosophical and theological theses, and of disputes about
disciplinary boundaries at the University.




Regius first came to Descartes' attention through a mutual friend,
Henricus Reneri (1593-1639), and almost immediately (in August
1638) began a correspondence with Descartes, which continued until
1645. In keeping with the custom at that time, he also presided as a
professor over disputations at the University (the equivalent of
research seminars, presented by students in the form of a defence of
theses that were composed by their supervisor). The texts of
disputations were normally printed and distributed in advance
of their oral presentation, and some rare copies of Regius'
disputations survive. These were initially concerned with medical
matters, such as a controversial disputation supporting Harvey's
theory of blood circulation (Regius 1640a). During the year 1641,
Regius considered publishing a book on natural philosophy, and
consulted both Descartes and Gisbertus Voetius (1589-1676), the
Professor of Theology and University Rector, about his plans. While
Descartes recommended that Regius develop his ideas in the form of
disputations, Voetius initially advised publication as a monograph,
possibly to avoid conflict with faculty members in theology or
philosophy. Regius adopted Descartes' advice and, in April 1641, he
began a series of disputations under the title *Physiologia sive
cognitio sanitatis* (Regius 1641a).




In November 1641, however, Regius initiated a new series of
disputations, *De illustribus aliquot quaestionibus
physiologicis* (Regius 1641b), in which he challenged Aristotelian
theses that were taught by his colleagues in philosophy and theology at
the University. This attracted a very negative response from Voetius,
who relied on Aristotelian philosophy to articulate his version of
Calvinist theology. The resulting lengthy controversy, during which
Regius lost the support of Descartes while simultaneously continuing
the conflict with Voetius, is discussed below. He published the first
edition of his book on natural philosophy under the title
*Fundamenta physices* (1646); Chapter XII, entitled *De
homine* (Regius 1646, 245-306), provided a synoptic
exposition of his controversial views about human nature.




Despite the bitter and extended controversy in which Regius engaged
with Voetius and the university authorities, he was later appointed Rector of
the University of Utrecht (a one-year honorary position) in 1649, and
again in 1662. He published an amended, enlarged edition of the
*Fundamenta physices* under the title *Philosophia
naturalis* in 1654, and a third edition in 1661. A French
translation appeared in 1687. He remained in Utrecht, except for a
brief period when he was captured as a hostage by the French army in
late 1673, and died there in 1679.


## 2. Dispute with Voetius and Descartes




Regius attributed his appointment at Utrecht University to his
familiarity with and support for Cartesian philosophy, which was
acquired initially by reading Descartes' first book, the *Discours
de la Methode ... plus la Dioptrique, les
Meteores et la Geometrie*, 1637 (AT,
vol. VI). Regius was sympathetic to Cartesian natural philosophy from
his initial reading and began to teach it to students, in private
lessons, and to dictate notes with a view to publishing his own
summary of natural philosophy based on Cartesian principles. This
objective was further encouraged when he read a copy of Descartes'
unpublished manuscript, *Le Monde* (AT, vol. XI), in 1641. The
natural philosophy of *Le Monde* rejected scholastic forms and
qualities as philosophical entities that contributed nothing to an
explanation of natural phenomena. There was a significant difference,
however, in a university such as Utrecht between not relying on
scholastic forms and criticizing them publicly. The transition from
the former to the latter, in the case of Regius, occurred in 1641.




While preparing *Physiologia* (Regius 1641a), Regius sent
advance copies of draft disputations to Descartes. Since they were
being presented in Utrecht as examples of Cartesian philosophy,
Descartes recommended various changes, both to avoid needless
controversy (for example, concerning blood circulation) and, more
importantly, to avoid accusations of heresy against himself or
Regius. Accordingly, he asked Regius to correct the thesis that the
human soul is three-fold, and to argue instead that "in human
beings the soul is one, namely a rational soul" (AT III,
369-74: Bos 2002, 63-69). During the year 1641, this
pattern of prior consultation with Descartes continued; Regius
accepted many of Descartes' suggestions, and incorporated them into
the final versions of the disputations.




In November 1641, however, the Utrecht professor inaugurated another
series of disputations in which he transgressed from medicine to
philosophy and, by implication, into theology, and thereby provoked a
controversy that continued for more than a decade. The most explicit
challenge to established Aristotelian teaching occurred when one of
Regius' students, Henricus van Loon, claimed in a disputation (8
December 1641) that the human mind and body are two distinct
substances which are joined together in an accidental union [*unum
per accidens*]. This thesis addressed, perhaps inadvertently, one
of the most contentious issues in Christian philosophy and
theology, *viz.* how to understand the unity of human nature,
as experienced by each subject, and at the same time defend the
traditional Christian beliefs in the immortality of the human soul or
mind and the resurrection of the body. The immortality of the soul
seemed to presuppose that the mind or soul was capable of existing
without the body and, for that reason, that it should be classified as
an Aristotelian substance. When Descartes was informed about this
disputation (when it was already too late to recommend amendments), he
responded that there was hardly anything more likely to offend or
antagonize theologians than to say that a human being "is an
accidental being" (AT III, 460: Bos 2002, 90-91).




Voetius, with the support of his professorial colleagues in theology,
replied to Regius by appending corollaries to his own theological
disputations, in December 1641, in which he accused proponents of the
'new philosophy' [i.e., Cartesianism] of a number of
errors: teaching that a human being is an accidental being, defending
Copernicanism, and supporting scepticism and irreligion by denying
substantial forms. In the course of these replies to Regius,
Descartes' name was also mentioned, and it was clear that Regius and
Descartes were perceived by Voetius as jointly attempting to undermine
the philosophical foundations of Calvinist theology as taught at
Utrecht. Initially, Descartes joined the debate merely by advising
Regius how to respond appropriately to Voetius. One part of that
advice was that Regius should have dealt with substantial forms as
Descartes had done in the *Meteores* (AT VI,
239): that he should not have rejected them explicitly but, instead,
should merely have omitted them as redundant to genuine
explanations.




However, Descartes also provided Regius with philosophical arguments
in reply to Voetius. He outlined a number of reasons why they were
jointly opposed to describing the human mind as a substantial form. To
say that human beings think because they have a 'thinking
form' explains nothing; secondly, those who postulate the
existence of substantial forms are unable to say more precisely what
they are, and thus they appeal to 'occult' realities that
they do not understand; and, perhaps most importantly, if the alleged
substantial forms of other natural phenomena were made redundant by 
developments in physics, there would be a danger that the spirituality
of the mind or soul would be compromised by similar advances in the
sciences. "All the arguments that are used to prove substantial
forms may be applied to the form of a clock, although no one claims
that it is substantial" (AT III, 505: Bos 2002, 115). Many of
Descartes' suggestions were incorporated, as amendments, into the
final version of the Regius' reply to Voetius, which appeared as
*Responsio, sive notae in Appendicem* (Regius 1642) on 16
February 1642. This further inflamed the situation. The following
month, Regius was forbidden by the University to teach natural philosophy in
public or in private, and was restricted to lecturing in traditional
medicine. The Academic Senate, with the approval of
the town council, also issued an official condemnation of Cartesian
philosophy. 




Descartes later joined the controversy in a more public way. He
included a letter to a French Jesuit, 'A Letter to Father
Dinet,' in the second edition of the *Meditations* (1642),
which was published in Amsterdam and thus became readily available to
his critics in Utrecht. While the letter to Dinet criticised Voetius
without naming him explicitly, Descartes' subsequent
*Epistola ad Voetium* (1643) went further by removing the
apparent anonymity of his target in the first letter. The complex
history of the next few years, which is summarized in Verbeek 1992,
included an official ban on the teaching of Cartesian philosophy by the
local authority in Utrecht (the Vroedschap), and the establishment by
the same authority of a committee of inquiry into the public
controversy that resulted from Regius' teaching of Cartesian
philosophy. The official report of the university committee on the
whole affair, to date, was published in 1643 as the *Narratio
historica* (Verbeek 1988, 71-123). Meantime, Voetius arranged
for one of his supporters, Martinus Schoock (1614-69), to write
an extensive critique of Descartes and Cartesianism, which was
published as *Admiranda methodus* (*The Admirable
Method*) in Utrecht in 1643 (Schoock 1643). *The
Admirable Method* listed, among the most serious charges against
Descartes, that he was an atheist and a sceptic, and it implied that he
deserved the same treatment as heretics, such as Giulio Cesare Vanini
(1585-1619), who had been burned at the stake in Toulouse. While
the authorship of this book, Descartes' replies to it, and
various legal charges of defamation and libel continued for some years,
Descartes concluded his contribution to the controversy with Voetius in
the *Apologetic Letter to the Magistrates of Utrecht*, which was
written in 1647 and sent to Utrecht in February 1648 (Bos 1999).




The intensity of the dispute between Voetius and those whom he
identified as proponents of a 'new philosophy' might
suggest agreement, at least between Descartes and Regius, on a common
philosophical position. In the course of the Utrecht controversy,
however, an irretrievable breakdown developed in relations between
Descartes and his former supporter. The epistemology favoured by
Regius, which reflected his disciplinary expertise in medicine and
physiology, left little room for Cartesian metaphysics. When he wrote
to Descartes, in July 1645 (AT IV, 254-6: Bos 2002,
189-90), that many reputable people believed that Descartes
secretly held views about the human mind and God that differed from
those that he published, and that he had discredited his philosophy by
publishing the *Meditations*, it seemed to Descartes that Regius
was endorsing the same criticisms that Voetius and Schoock had
made earlier. The following year, against Descartes' advice, Regius published
the *Fundamenta physices*, in which he rejected a number of
Descartes' metaphysical theses and thereby incurred the wrath of
his former mentor. The French philosopher denounced Regius, as
misrepresenting his philosophy, in the preface to the French edition of
the *Principles of Philosophy* (1647). Regius planned to reply
by adding twenty-one Corollaries to a medical disputation that was scheduled
for 2 October 1647, concerning a forty-year old man who had
inflammation of the feet and legs (Regius 1647b). These Corollaries
included three of the more contentious claims about the mind and God
that Regius had earlier removed from *Fundamenta physices* in
an unsuccessful attempt to appease Descartes. Although the
disputation was banned by the University rector, Regius had already
disseminated copies of the text, including the controversial
*Corollaria*. Descartes replied with *Notae in programma
quoddam* (1648)--i.e., *Notes on a Certain
Manifesto*--to which Regius replied with an extended version
of the *Corollaria*, in *A Brief Explanation of the Human Mind
or the Rational Soul, in which is explained what it is and what it may
be* (Regius 1648).




The main source of disagreement between Descartes and Regius was their
respective accounts of human nature. This had emerged as early as
1640, when Regius commented on a draft of the *Meditations*,
and more explicitly in the disputation (December 1641), which proposed
the thesis that the human mind and body are united in an
'accidental unity.' In July 1645, when Descartes responded
to a draft of the *Fundamenta*
*physices*, he argued that Regius had gone too far in the
opposite direction by making the union of mind and body so close that
the mind was merely a 'mode' of the body--which was
an even greater error than his earlier mistake (AT IV, 250: Bos 2002,
188). This issue, of how to conceive of the mind and body as united
and yet separable, dominated the series of claims and counter-claims
made by both philosophers up to 1648. Between 1648 and February 1650,
when Descartes died, the two philosophers did not engage directly with
each other. Descartes discussed a range of questions with Burman in
1648, and in response to one query he provided another reminder of the
extent to which Regius allegedly misunderstood his
metaphysics. "As far as Regius is concerned ... in physics
he has always been anxious to follow the views of the author [i.e.,
Descartes] ... whereas in metaphysics he has done everything
possible to contradict the author's views" (Descartes CB, 38:
AT V, 170). Descartes' last publication, *Les Passions de
l'Ame* (1649: AT, vol. XI), may be read as his final attempt to
address the source of their disagreement, and Regius 1650 may be seen
as Regius' response (Verbeek 2017).


## 3. Theory of Knowledge and Philosophy of Science




Once Regius had publicly established his independence vis-a-vis
Descartes in 1647, he began to publish a theory of knowledge that
reflected his medical interests (Bellis, 2013) and complemented the account of human
nature that he adopted in opposition to his former mentor. The result
was an epistemology that was closer to that which appeared, almost
four decades later, in Locke's *Essay* than anything that he
had read in Descartes. This transition had begun while drafting
*Fundamenta physices*, in which Regius denied the need for any
innate ideas, including even the idea of God.



> 
> 
> The mind, in order to think, does not seem to need innate ideas,
> images, notions or axioms; rather, its innate faculty of thinking is
> sufficient on its own in order for it to complete all its thinking
> activities. This is evident in the perception of pain, colour, taste,
> and other similar perceptions, which are genuinely perceived by the
> mind, although none of those ideas is innate. Nor is there any reason
> why some ideas rather than others should be innate by their very
> nature. (Regius 1646, 251)




Regius supported this with the claim that a young person who had not
been educated would, if asked, deny knowledge of things of which they
had no experience (1654, 251). Since an innate idea of God was central
to the Cartesian project, Regius made his own position explicit by
also denying that the idea of God is innate: "Even the idea of
God ... does not seem to be innate in us, but is produced in us
initially by observing things or is passed on to us by others"
(Regius 1646, 252).




Regius rejected the innateness of ideas for the same reason that
Descartes had rejected various scholastic entities: it was redundant
to an adequate explanation of thinking.



> 
> 
> It is evident from the perception of titillation and of pain, of
> sounds and light, which arise from the motion of an objective body,
> that the stimulation of the senses does not require intentional
> species, nor spiritual qualities, nor any other unintelligible things,
> but that local motion alone and its variations suffice for this
> purpose. (Regius 1646, 253-4)




He repeated the denial of innate ideas in the Corollaries of 1647:
"The mind does not need innate ideas, or notions, or axioms; its
faculty of thinking alone is enough for it to perform its own
actions" (Regius 1647b, Corollary 12). Descartes' reply, in
the *Notae* (Descartes 1648), conceded the validity of Regius'
claim by accepting that their disagreement was merely verbal: "I
never wrote or claimed that the mind needs innate ideas which are
anything other than its faculty of thinking" (AT VIII-2,
357). Regius subsequently generalized his claim, in
*Philosophia naturalis* (1654: 335); apart from what
is revealed in Scripture, he argued, all thoughts originate in sensory
experience.



> 
> 
> All those [i.e., conscious acts], which do not originate in divine
> revelation, are sensations or derive their origin from sensation. For
> we cannot will, judge, or remember anything, nor imagine anything, nor
> otherwise perceive anything, unless some idea of that thing had been
> immediately or mediately produced in us by sensation, or subsequently
> triggered and presented to the mind.




This unequivocally empiricist claim was compounded by the denial of
any 'pure intellect.' Regius had never appealed to a
'pure understanding' as a source of ideas, and had often
asserted the 'organic' connection between mind and
body. He had defined 'organic', in the first of his
*Physiologia* disputations (Bos 2002, 200), in contrast with
'inorganic' as almost a synonym for 'bodily
dependent'. Organic parts of the body, such as the nerves,
veins, or muscles, required a certain physical conformation of the
body in order to perform an action. When applied to the mind,
therefore, the term 'organic' suggested that the mind
could perform mental operations only on condition that various bodily
parts functioned appropriately at the same time.




These early hints, which were short of an outright denial of a pure
understanding, were interpreted correctly by Descartes in 1647. Regius
had defined perception in his banned disputation as 'sensation,
remembering and imagining' (Regius 1647b: *Corollaries*,
no. 18), and Descartes concluded that he was implicitly denying the
reality of pure understanding: "One could draw the conclusion
that he does not accept any pure understanding, that is, an
understanding that is not concerned with physical images; and
therefore he thinks that we have no knowledge of God or the human
soul, or of other non-physical things" (AT VIII-2, 363). Regius
confirmed that interpretation in his reply: "The human mind,
although it is a substance that is really distinct from the body, is
however organic in all its actions while it is in the body ... it
cannot perform any of its actions without bodily organs" (Regius
1648, 10). Perhaps he had been applying Descartes' previous advice
about substantial forms--to ignore them rather than deny their
existence explicitly--when he refrained from denying a pure
understanding in his earlier writings. By 1654, however, he was ready
to publish an explicit denial: "Since ... we are able to
explain all its operations [i.e., those of the mind] by those
faculties of the intellect, which were already explained, there is no
need to add to them a pure intellect or anything similar as something
that is distinct from them" (Regius 1654, 404). Nonetheless, the
denial of a pure intellect did not imply denying that human beings
have an idea of God; the latter is acquired in the same way as other
ideas, namely by using an intellect that is organically dependent on
the human body.




Since all ideas originate in sensation, Regius had to acknowledge that
we often come to have ideas that originate from imagined realities and
that, in the case of veridical and non-veridical perceptions, the
subjective experiences are often sufficiently similar that one cannot
be certain of a correspondence between ideas and reality. In contrast
with the Cartesian rational defence of the reliability of our
cognitive faculties, Regius claimed that one must rely on Scripture
for this purpose (Regius 1646, 249). However, Scripture does not
guarantee that our perceptions are always certain. This issue was
particularly acute when the realities in question were unobservable
(Regius 1646, 4). Regius thought it was obvious that physical bodies
are composed of insensible particles, and that their description is a
matter of hypothesis. When faced with the task of explanation,
therefore, he took refuge in some of the central themes of Cartesian
natural philosophy. He borrowed the 'fable' used by
Descartes in *Le Monde*, that the natural world is composed of
one matter, that it is divided into parts of various sizes and shapes
in motion, and that all natural phenomena may be explained by the laws
of nature that apply to those parts (Regius 1646, 4).




This is the positive side of Regius' account of explanation. The
negative side, as with Descartes, is a rejection of forms and
qualities as understood by scholastic philosophers. For example,
Regius had argued in 1641 that one does not explain how the heart
works by introducing a 'sentient and vegetative soul.' He
commented: "That is saying nothing more than this: that the
heart is moved by whatever is the principle by which we live, sense
and move (for that is how the soul is defined), which begs the
question as to what that is" (Regius 1641b, Disputatio VI,
Corollary, par. V). Such an explanation of the heart's motion is
similar to explaining the motion of a clock by 'a faculty of a
certain kind.' Regius proposed instead that we think of the
world as a great machine, and that we explain natural phenomena by
hypothesizing how that machine works.



> 
> 
> The world, therefore, is a great machine and each of its parts is a
> small machine, in which it can be observed and easily demonstrated
> that there is nothing other than motion or rest, together with size,
> position, and shape (I wish to make a unique exception of our soul).
> However, I regard the forms which are said to be substantial and
> likewise qualities of a corporeal substance, whether occult or
> manifest ... as mere non-entities, which conceal our ignorance of
> causes. (Regius 1641b, para. ix)




It follows from this kind of speculative reasoning about the
unobservable inner structure of natural phenomena that scientific
knowledge is only more or less probable. If we imagine a likely cause
of some phenomenon as a configuration of unobservable particles and,
despite further research, cannot discover another possible cause that
is either as likely or more likely, we should rest content with the
first hypothesis until something better is discovered.



> 
> 
> When any question in physics is proposed for resolution, one should
> first think of an intelligible cause by which the effect that is
> observed in the proposed problem could be explained intelligibly and
> appropriately. One should then look to see if a more appropriate or
> equally appropriate cause can be found. If a more appropriate cause is
> found, it should be preferred. If one that is equally appropriate is
> found, it should be treated as equally confirmed. However, if none is
> found that is equally or more appropriate, the solution found should
> be adopted until a better or equally good one is found. (Regius 1654,
> Epilogue)




The acceptance of probable hypotheses as the best available was a
significant revision of a model of human knowledge, originally derived
from Aristotle, according to which genuine knowledge must achieve the
degree of certainty that is provided by intuition and demonstration
(Clarke 2011). While other natural philosophers, even at the
conclusion of the seventeenth century, refused to classify
explanations of natural phenomena as knowledge, because they were
uncertain, Regius had accepted in mid-century that such uncertainty is
inevitable except in the case of divine revelation. "Although,
apart from divine revelation, there is no apodictic certainty about
things that exist outside our mind, but only moral or probable and
likely certainty, that is still sufficient to perform adequately and
to control all the activities of human life, since nothing more is
required for them apart from moral or probable truth or the certainty
and likelihood of knowledge" (Regius 1654, 351).


## 4. Theory of Human Nature




Regius acknowledged objections, as early as 1641, to scholastic
descriptions of human nature, according to which the human mind or
soul was the substantial form of the body that was capable, after
death, of existing on its own. As indicated above, he accepted
Descartes' general objections to the intelligibility and explanatory
usefulness of substantial forms. It was unclear to him, therefore, how
such forms could be used to describe or explain human nature. His
reservations about substantial forms were articulated in the draft
copy of *Fundamenta physices*, which he had sent to Descartes prior
to publication. The latter's objections caused Regius to delete three
of the more controversial claims about God and the soul from the
published text. However, they re-appeared in the Corollaries of Regius
1647b, and in the second edition of the text, *Philosophia
naturalis* (1654).




Descartes had defined material substance in terms of its principal
attribute, extension. Regius endorsed that position when he rejected
any distinction between a corporeal substance and its extension.



> 
> 
> There is no need to imagine that there is a substance in a body in
> which extension exists, and that it is something which in its nature
> or in reality is distinct from extension, because realities should not
> be multiplied without necessity. Extension does not need such a
> subject, *as something that is distinct from itself*; but it is
> capable of existing on its own. (Regius 1646, 2; the phrase in italics
> was added in Regius 1654, 3.)




One would expect the same logic to apply to thinking, as an attribute
that does not require a distinct substance. Nonetheless, Regius initially
stopped short of saying that. While accepting that divine revelation
teaches that the mind is a substance, he emphasized that it is
organically united with the body. "The human mind, although it
is a substance that is really distinct from the body, is however
organic as long as it exists in the body" (1646, 246). The mind
was also described as being "united most closely with the body
in one substance" (Regius 1646, 248).




However, once he had been rebuked publicly by Descartes, Regius was
free to express himself more explicitly. In the Corollaries (Regius
1647b), he wrote:



> 
> 
> As far as the nature of things is concerned, it seems possible for the
> soul to be either a substance or a mode of a physical substance. Or,
> if we follow some other philosophers [i.e., Descartes] who stipulate
> that extension and thought are attributes which are present in certain
> substances as their subjects, then since these attributes are not
> opposites but are just different from each other, there is nothing to
> prevent the mind from being an attribute of some kind which applies to
> the same subject as extension, even though neither attribute is
> included in the concept of the other (thesis 2).




This is further developed in Regius 1654, in which he argues that the
concepts of extension and thinking are not incompatible and,
"accordingly, they [the attributes] may both be present in the
same simple subject" (337). This anticipates the question (and
the implied answer) that was discussed by Locke, in the *Essay*
IV, iii, 6, whether God may have superadded thought to a material
substance. In the version proposed by Regius, there are no reasons why
it would be impossible for a material entity to have the property of
thinking because, although thinking and extension have nothing in
common, they are not incompatible.



Regius also included in the *Corollaries* two other theses that
were critical of Descartes and had been omitted from
the *Fundamenta Physices*: in thesis 3, he claimed that 'those
who assert that we conceive of the human mind clearly and distinctly
as necessarily and really distinct from the body are mistaken', and in
thesis 13 that 'all common notions that are engraved in the mind
derive their origin from the observation of things or from
tradition'. Regius also claimed that these conclusions from experience
and reason, within the scope of natural philosophy, were consistent
with thesis 4, that divine revelation provides indubitable evidence
for belief that the human mind is a distinct substance.




These early statements in thesis form of criticisms of Descartes were
subsequently expanded in Regius 1654. Many of Descartes'
contemporaries had understood him as arguing that there must be a real
distinction between mind and body because it is possible to doubt
having a body although it is impossible to doubt having a mind. Regius
addressed this argument explicitly:



> 
> 
> The fact that we can doubt the body but cannot possibly doubt the mind
> does not prevent the mind from being a mode of the body. That merely
> proves that, while we are doubting the body, we cannot say with
> certainty that the mind is its mode. In the mean time, since the body
> about which we doubt may nevertheless subsist and since there is no
> reason why it may not be modified by the mind--which is evident
> from what has been said earlier--this is enough to show that the
> mind of which we are certain may be a mode of that doubtful body.
> (1654, 338)




Given Regius' disagreement with Descartes about the concept of a human
mind, and his rejection of innate ideas, it was not surprising that he
also rejected the ontological argument for God's existence that
Descartes had proposed in the Fifth Meditation (AT VII, 65). The Dutch
philosopher accepted that the "actual and necessary existence of
God is comprehended in the idea or concept of God." He argued,
however, that it does not follow from that concept that God exists
actually and necessarily. "It merely follows that, if he existed
anywhere, he would exist necessarily rather than contingently or that,
in that case, he would not be capable of not existing" (Regius
1654, 357).




One could interpret the gradual dissociation between Regius and
Descartes, and the rejection by the former of the latter's
metaphysics, as an endorsement by Regius of one dominant feature of
the Cartesian enterprise, namely, the new natural philosophy that was
inaugurated in *Le Monde*. Regius had followed the logic of
Descartes' natural philosophy by denying any explanatory value in the
hypothesis of souls in animals. "Since indeed the actions of
beasts ... can be explained simply by the disposition of their
spirits and other bodily parts, and since entities should not be
multiplied without necessity, we have no reason to attribute any
intellect to beasts" (Regius 1646, 241-42). He might have
followed the same logic in the case of human intelligence, had he not
been restrained by his understanding of the teaching of Scripture, and
his apparent acceptance on biblical grounds that the human mind is an
immaterial substance. Rather than endorse Cartesian metaphysics, as
found in the *Meditations*, which seemed to return its author
to the scholastic philosophy that he explicitly rejected, Regius
struggled with the competing demands of Scripture and the logic of
Cartesian natural philosophy. The challenge of this conjunction is
evident in Regius' repeated attempts to describe the unity of mind and
body 'in a single substance' (1646, 248) while maintaining
that the mind is "something that is really distinct from the
body and is separable from it in reality" (1646, 246). This
epistemic dualism, to the effect that we acquire some beliefs from
natural knowledge and others from Scripture, has been understood, by
some readers, as intellectually dishonest and as a typical libertine
camouflage for cryptic materialism (Hallyn, 2005). But, as Bos (2013)
shows, Regius may have accepted the limits of human knowledge, based
on reason and experience, and also adopted a commonplace understanding
of the certainty of Scripture. By doing so, he became a reluctant
dualist without endorsing Cartesian metaphysics.