A typical textbook account of the philosophy of mind in the seventeenth century goes something like this. Descartes believed in two kinds of stuff, mental stuff and material stuff, substances distinct in nature that go together to constitute a single human being. But Descartes also took it for granted that these two substances were capable of genuine causal interaction, that minds can cause bodily events, and that bodies can cause mental events, i.e., that acts of will can genuinely cause changes in the state of the human body, and that the state of the sensory organs and the brain can cause sensation and imagination in the mind. But, the story goes, Descartes went astray here and vastly underestimated the philosophical problems inherent in his position. Descartes, it is claimed, repressed, or even worse, simply ignored the central question his position raises: How is it even possible that an immaterial substance, like the mind, could conceivably act on an extended substance like the human body? According to the standard account, later philosophers recognized the inherent unintelligibility of Descartes' position and started one of the largest cottage industries in the history of philosophy, the attempt to provide satisfactory solutions to the mind-body problem, intelligible accounts of how mental and physical events are related to one another. Realizing the unintelligibility of the doctrine of causal interactionism, this cottage industry produced such noteworthy products as occasionalism, dual-aspect theory, pre-established harmony, and so on, all in the attempt to fill in the gap in Descartes' dualist program.

This general outline can (and has) been challenged; the actual history of philosophy is much richer than any of its rationalized reconstructions.

One of the central doctrines of Descartes' metaphysics was his division of the created world into two kinds of stuff: mental substance whose essence is thought and material substance whose essence is extension. And one of the central problems that later philosophers had with Descartes' doctrine was understanding how these two domains, the mental and the material, relate to one another. Descartes' solution was to claim that these two domains can causally interact with one another, that bodily states can cause ideas, and that volitions can cause bodily states. But this claim raises a number of serious questions. The most obvious problem arises from the radical distinction that Descartes draws between the two domains and from our difficulty in conceiving how two sorts of things so different could ever interact with one another. As the Princess Elisabeth complained to Descartes, “it is easier for me to concede matter and extension to the mind than [it is for me to concede] the capacity to move a body and to be affected by it to an immaterial thing.” Though the story is complex, it is generally held that this problem led later in the century to the doctrine of occasionalism, in which the causal link between mind and body was held to be not a real efficient cause but an occasional cause. Thus, it was claimed, it is God who causes ideas in minds on the occasion of appropriate events in the material world and events in the material world on the occasion of an appropriate act of will. The causal link between mind and body remains but is reinterpreted as an occasional causal link, a causal link mediated by God.

René Descartes (1596–1650) aimed to sweep away the past, and start philosophy anew. Much of what made Descartes important for his contemporaries, and for us as well, concerns the contents of his philosophy. Descartes' philosophy was directed squarely against the Aristotelian philosophy taught in the Schools of his day. For the Aristotelians, all cognition begins in sensation: Everything in the intellect comes first through the senses. Descartes' philosophy, on the other hand, emphasizes the priority of reason over the senses. Furthermore, Descartes substitutes a purely mechanical world of geometric bodies governed by laws of motion for an almost animistic world of Aristotelian substances with innate tendencies to different kinds of behavior. These original doctrines, together with his work in metaphysics, optics, mathematics, the theory of the passions, among other areas, made Descartes a central figure in his age.

But in this essay I would like to concentrate on something different. Descartes opposed himself not only to the content of the philosophy of the Schools, but to their very conception of what knowledge is and how it is to be transmitted. Connected with the new Cartesian philosophy is a genuine philosophy of education, a conception of the aims and goals of education very different from the one that dominated the School where Descartes himself had been educated as a youth. My project in this essay is to tease out some aspects of this philosophy.

Rejecting Authority

Let us begin with one of Descartes' most important texts, the Discourse on the Method of Rightly Conducting One's Reason and Seeking the Truth in the Sciences, published in 1637 as the introduction to three scientific texts, the Geometry, the Dioptrics, and the Meteors.

Descartes is usually classed among the rationalists, those philosophers who privilege reason over experience. And indeed he belongs there. On the other hand, though, Descartes was also very interested in experiment. The Dioptrique and Météores make a number of references to Descartes' experiments; the Discours discusses the importance of experiments at some length. In the Principia, written starting in early 1641 and published in 1644, Descartes refers to a number of experimental results to support his views, most visibly in the discussion of the magnet. And at the end of that book, he goes so far as to suggest that his vision of the world is ultimately supported by the fact that it is capable of explaining observed phenomena, and nothing more. Where is the real Descartes? Is he mathematician or experimenter? rationalist or empiricist?

This is the larger question that I would like to explore in this essay. But I would like to address it in a rather particular and somewhat special way. Generally, discussions of Descartes' views about knowledge and experience concentrate on texts like the Meditations, and on issues concerned with knowledge of the kinds of grand questions that he takes up there, the knowledge of self, body, the distinction between mind and body, God, and so on. What I want to focus on is something much more mundane. The water we drink every day has a nature, from which follow certain well-known properties; water is wet and liquid at room temperature, solid when very cold, quenches thirst, admits light, but causes certain illusions, like the famous bent-stick illusion. All of this is somehow connected with its structure.

The Sixth Objections, like the Second Objections, were collected by Father Marin Mersenne, and purport to represent the views of the group of philosophers and theologians who belong to the so-called Mersenne circle. The very first objection that Mersenne and his friends make to the Meditations in the Second Objections concerns the real distinction between mind and body; Mersenne and his friends simply do not understand how Descartes' arguments exclude the possibility that thought is not a kind of motion, and why a body cannot think (AT VII 123). Descartes, of course, attempts to answer this question in the Second Replies (as well as in the Third and Fifth Replies), but evidently not to Mersenne's satisfaction. For in the Sixth Objections, the very same question is raised yet again (AT VII 413). Mersenne goes on to suggest that even the Church Fathers believed that thought “could occur by means of corporeal motions” (AT VII 413). The Sixth Objections ends with an appendix and a letter “from some philosophers and geometricians to M. Descartes” in which these very same doubts are voiced again:

It is generally recognized that knowledge for Descartes is the clear and distinct perception of propositions by the intellect; knowledge in the strictest sense is certain, indeed indubitable, and grounded in the purely rational apprehension of truth. But it is also generally recognized that Descartes was a serious experimenter, at least in his biology and his optics, and that in these areas, at least, he seemed to hold that knowledge requires an appeal to experience and experiment. Writing, for example, in Part VI of the Discourse on Method, Descartes laments the fact that he has neither the time nor the resources to perform all the experiments (expériences) necessary to complete his system, and calls upon his readers to “communicate to me those that they have already made, and to help me in performing those which remain to be done” (AT VI 65). (One can see in the Discourse a clear anticipation of an important later literary form, the grant application.)

To the twentieth-century philosopher this looks a bit puzzling: How can Descartes be both a rationalist, who sees knowledge as deriving from the intellect, and an experimentalist, who sees experiment and observation as essential to the enterprise of knowledge? This is the puzzle I would like to address in this essay. I shall argue that not only is there no contradiction here, but that the appeal to experience is an essential part of the method for constructing a deductive science. We shall begin with a brief account of Descartes' procedure for constructing his science, his method.

My interest in Descartes was originally piqued when, as a graduate student, I had to assist in an introduction to philosophy. The Descartes I was asked to teach the students didn't make much sense to me; I couldn't figure out his point of view, why he was asking the kinds of questions he was asking, and why he was giving the kinds of answers he was giving. Something about his larger intellectual context seemed to be missing. But even then I knew that Descartes was deeply involved in the physical sciences of his day, and even without knowing exactly what Cartesian science meant, I had a deep suspicion that it was somehow connected with the philosophical writings I was teaching my undergraduates, the Meditations and the Discourse on the Method. At the time I was also very interested in the latest currents in contemporary philosophy, particularly the philosophy of Quine. Quine's enormously influential “Epistemology Naturalized” had just appeared, and everyone was talking about a more general naturalization of philosophy and the intimate connection between philosophy and the sciences. That gave me all the more reason to turn to Descartes and his contemporaries, who, in a sense, took it for granted that there was a continuum between what we call philosophy and what we consider the sciences.

And so I undertook a serious study of Descartes' science, as well as that of his contemporaries. This led me to a number of interesting observations. I came to see that Descartes' thought must be understood in the context of the attempt to reject Aristotelian physics, and replace it with a different kind of physics, one grounded in a mechanistic conception of nature.

The serious student of Descartes' philosophy must deal with the fact that Descartes' metaphysics is presented in a number of different ways in a number of different works. While the Meditations ought to be regarded as the authoritative text, it is important to account for the sometimes significantly different versions of the philosophy that Descartes presents in the Discourse, the Principles of Philosophy, the Search After Truth, and in numerous remarks scattered throughout the correspondence. In this essay we shall examine one attempt to explain the principal differences between two of these works: the Meditations and the Principles. It is often claimed that these differences can be explained by the fact that the Meditations are written in accordance with the analytic method, whereas the Principles are written in accordance with the synthetic method. We shall argue against two somewhat different versions of this thesis. Although we have no counter-thesis of comparable power or simplicity to offer, we shall suggest some ways of understanding the relations between these two central works that better reflect the texts and what appear to be Descartes' intentions.

The main source for our understanding of Descartes' distinction between analysis and synthesis is the difficult thought often cited passage at the end of the Second Replies (AT VII 155–156). In the Second Objections, Descartes is requested to present his argument in more geometrico, with the full apparatus of definitions, postulates, and axioms (AT VII, 128). Descartes complies with this request in the Geometrical Appendix which follows his Second Replies where he provides a geometrical exposition of some of his arguments. But first Descartes gives a general discussion of the geometrical method of presentation.

Introduction

In his important and influential book, How Experiments End, Peter Galison discusses how it is that scientists decide when a given experiment is finished and when the supposed fact that it purports to establish can be accepted as fact and not a mistaken reading of the apparatus, not a result of a malfunctioning piece of equipment, not a misinterpretation of a given observation, and so on. This epistemological question – the transition between individual observations, individual runs of a complex experiment, and the experimental fact that they are supposed to establish – is a matter of some discussion in the recent literature in the history, philosophy, and sociology of science. It is this question that Galison (and others) have called attention to that I would like to explore in this article.

What strikes me as interesting here is that the very question under scrutiny has a history; while, in a sense, the question has been with us as long as people turned to experience to try to figure out how the world is, people were not always interested in or aware of the question, and when they were, the answers that they suggested were not always the ones that we find most comfortable now. That is what will interest me here, the history of the notion of an experimental fact, if you will, or, as Lorraine Daston has dramatically dubbed it, the “prehistory of objectivity.”

The doctrine of occasionalism was, of course, central to seventeenth century metaphysics. On this widely held view, the changes that one body appears to cause in another on impact, the changes that a body can cause in a mind in producing a sensation, or that a mind can cause in a body in producing a voluntary action are all due directly to God, moving bodies or producing sensations in minds on the occasions of other appropriate events. And so, on this view, the tickling of the retina and subsequent changes in the brain are only the “occasional causes” of the sensory idea I have of a friend in the distance; the real cause is God, who directly moves my sense organs when the light approaches them, moves the parts of the brain when the sensory organs are moved, and then produces the sensory idea I have in my mind of another person's face when my sense organs and brain are in an appropriate state. Similarly, it is God who is the actual cause of my arm's movement when I decide to raise it to wave; my volition is only an occasional cause.

Now, occasionalism was widely held among many of Descartes' followers; it can be found in various forms in Clauberg, Clerselier, Cordemoy, La Forge, Geulincx, and, most notably, in Malebranche. And throughout its seventeenth-century career it is closely associated with Descartes' followers. But to what extent is it really Descartes' own view? To what extent is it fair to attribute this view to the founder of the Cartesian school? This is the question that I shall explore here.

Of the seven sets of objections to the Meditations, two stand out as being a bit different, the Second and the Sixth. In every other case we can identify one person, a philosopher or a theologian, who is the author of those objections. In the case of the Second and the Sixth, though, we are dealing with objections that have been collected by one person, the ever-present Father Mersenne, but that purport to represent the work of a number of other scholars, who remain unidentified. For most purely philosophical purposes, this does not matter a great deal; after all, an idea is an idea, whoever happens to have it, and if what is important is just the confrontation of ideas with one another, then the particular identity of the authors in question, those who contributed to these two sets of objections, is relatively unimportant.

But for those of us with a more historical approach to the texts, this is an unfortunate gap. First of all, it is intrinsically interesting from an historical point of view to know who may have contributed to the drafting of these objections. But more important, in order to understand the objections, their meaning and import, it is very important to know something about their authors. In particular, I shall argue that, behind the scenes in the Second Objections and Replies, there is not merely an author but a text that is important for understanding what Descartes is doing, a text that is implicitly referred to in the Second Objections and is the direct object of Descartes' reply in the geometrical presentation of the arguments that follows the Second Replies.

Descartes opens Meditation I with his persona, the meditator, reflecting on the project to be undertaken. Descartes writes:

And with this, the project has begun. Descartes' meditator quickly begins by rejecting the commonsense epistemological principles on which everything he formerly believed rested, and quickly sets about putting the world back together again. Of course, one of the central projects undertaken in this connection must be the replacement of the epistemological principles rejected with new, more trustworthy principles. Just as Descartes' meditator undermined his former beliefs by undermining the epistemology on which they were based, he will rebuild his world by rebuilding its epistemology. New epistemological principles thus seem to be the very “first foundations” on which he will build something “firm and lasting in the sciences.” But an obvious question to raise about this, the opening sentence of the Meditations, and about the project that follows out of it, is why? Why does Descartes believe it necessary even once in life to rebuild all of our beliefs in the way he suggests? Why does Descartes feel called to such an epistemological project?