Plan of the Book

This is a book about how two French philosophers of the seventeenth century, Rene Descartes and Nicolas Malebranche, along with some of their countrymen, deal with a crisis in the theory of perception. That century witnesses the demise of two central elements in philosophical thinking about sensory perception, elements that go back at least to Aristotle. First is the view that the proper objects of each sense—qualities such as color and sound—are real members of the mind-independent world. The common sensibles such as size, shape, and motion are perceived by perceiving the proper sensibles. When color and the rest are ejected from the mind-independent world, philosophers find themselves compelled to offer a totally new account of how it is that the mind comes to the common ones. Is it by inferring size, shape, and motion on the basis of, say, color or felt pressure? Or is it a purely automatic operation, accomplished by divine decree? Our experience of the proper sensibles becomes problematic as well. How is it that a sensible quality like color gets ‘localized,’ that is, experienced as being on the surfaces of the body that causes it?

The crisis also owes its origin to a second development. Since Galen, philosophers such as Ibn al-Hacen and Roger Bacon contributed to a unified program known as the ‘Baconian synthesis.’ This view posits species that pass into the eye and through the ventricles of the brain, ultimately uniting with species from the other senses in the organ of the common sense. Johannes Kepler in effect demolishes the Baconian synthesis. A new story has to be substituted; and it must be one that respects the new austerity of the world beyond the mind.

Descartes’s ambitious early work aims to develop a fresh account of the physiology of perception. On this view, there is no material species passing into the pineal gland, which Descartes takes to be the organ of the common sense. Instead, motion reaches into the brain and sketches an image on the pineal gland. Armed with this notion of a brain image, Descartes believes he can explain the ‘perception’ of non-human animals and even human beings when their minds are not attending to their environments.

It would be a shame to have developed such a sophisticated account of mindless ‘perception’ only to drop it when the activity of a human mind experiencing its environment is at issue. The early Descartes believes he can use his new physiological account to explain how we cotton on to the common sensibles in a world devoid of proper ones. On this view, or so I shall argue, it is by being aware of the image sketched on the pineal gland that we become aware of objects in the world. The purely corporeal brain image functions as a representation because it resembles an object in the subject’s environment. When the mind is aware of the brain, it thinks through this image out to the object. For this reason, the early Descartes, and even the Descartes of the second Replies, does not hesitate to call the brain image an ‘idea.’

Rather than struggle to read away this part of Descartes’s view, I show how it makes sense in its context. For it is a mechanistic counterpart of Aquinas’s position, according to which one thinks about singular things by convertendo se ad phantasmata, ‘turning toward the phantasms,’ which are purely corporeal images. Descartes’s version of this conversio doctrine is not a bold and counterintuitive innovation but rather an effort to hew as closely to tradition as his ontology will allow.

Even in his earliest stages, however, Descartes thinks human vision, when accompanied by attention, is supplemented by a variety of mental calculations, which he calls ‘natural geometry.’ (As we’ll see, in offering two means of judging distance, one purely corporeal and one involving reasoning of some kind, Descartes is following the well- worn Baconian path.) Natural geometry is not necessary for perception, whether merely mechanical or mental. Non-human animals and inattentive humans navigate their environments successfully without it. Thus Descartes’s early view allows him to give a univocal explanation of what I call the ‘overlap region,’ that set of behaviors exhibited by both animals and inattentive humans. What is more, the same physiological elements that underwrite ‘perception’ in the overlap region are exploited by the theory of mental perception.

As time goes on, Descartes becomes aware of the manifest shortcomings of the brain image. That image is subject to numerous distortions and explains far less about the phenomenology of perception than Descartes needs it to. As a result, his middle and later work eschews the brain image. While we still need to be aware of motions in the brain or in the eye, the brain image drops out of the account as a mere side effect of those motions. In its place, Descartes relies more heavily on the natural geometry announced in the early work. The prospect of giving a univocal account of the overlap region dims. If the human perception of distance and position requires the exercise of natural geometry and is not merely supplemented by it, it is hard to see how Descartes proposes to account for the apparently quite sophisticated ‘perceptions’ of non-human animals. What is more, Descartes now needs to provide new answers to our questions above: if it is not by being aware of a brain image that one becomes aware of the size, shape, and position of bodies outside the mind, how, exactly, is that accomplished? And how does the mind come to localize the sensible qualities it experiences?

On Descartes’s final view, the mind’s ideas and sensations are simply triggered when it becomes aware of motions in the brain. They then have to be projected (‘rapporter,’ ‘referred’) on to bodies. Although Descartes never drops the view that the mind is aware of the brain when it undergoes sensory experience, the explanatory power of the brain is at its nadir in this stage. That makes it all the more difficult to see just on what basis the mind is going to refer its sensations and ideas to objects in the world.

It is at this array of views that Nicolas Malebranche levels his most intriguing argument against Descartes, what I call the ‘selection argument.’ In short, Malebranche thinks that nothing about the physiology of the brain or eye can direct the mind to summon this or that idea or sensation. And nothing can guide the mind in projecting its sensations on to bodies. It seems obvious to Malebranche that Descartes’s picture is backwards: the mind has to use its sensations of color to individuate bodies in the first place. It will take some work to tease out just which of Descartes’s views Malebranche has in mind at various stages of the selection argument and judge its effectiveness. I shall argue that the selection argument poses a genuine challenge for each iteration of Descartes’s views.

Although there are some orthodoxies among the Cartesians—chief being the distinction between mind and body—their theories of perception are a heterogeneous bunch. Louis de la Forge and Robert Desgabets build on Descartes’s final and earliest works respectively, while Pierre-Sylvain Regis is most influenced by Descartes’s middle period. On Regis’s view, we sense color and sound and the rest but only imagine size, shape, and motion: we have to discern the invisible outlines of objects by clothing them in the mind’s qualities. For Desgabets, the mind has to inspect a brain image. The case ofAntoine Arnauld is especially intriguing in this light. I argue that his departures from Descartes are less important than they seem, and that he, too, feels the sting of Malebranche’s selection argument.

But if Malebranche has a keen eye for weaknesses in the Cartesian position, his own early view is vulnerable to much the same objection. By peeling away the layers of later additions and revisions, we’ll find the first edition of The Search after Truth introducing the doctrine of natural judgments. These judgments perform a variety of functions: assessing distance and position, correcting for optical illusions and distortions, and explaining the localization of sensible qualities.

Within three years, however, Malebranche changes his mind. The second edition of the Search robs these natural judgments of their force and indeed ownership: they are now done for us by God. Their nature is altered as well: instead of being judgments in any interesting sense, they are now simply compound sensations.

Later developments travel even further from Descartes’s work. Malebranche replaces the picture of the mind coming into contact with a single idea of a body, which in turn represents that body. Instead, Malebranche posits a single, entirely uniform idea of extension in God’s mind, which he calls ‘intelligible extension.’ In the end, I argue that, as far as sensory perception is concerned, Malebranche does not even believe that there are ideas, in any interesting sense. Put differently, he aims to return the notion of an Idea to its proper Platonic context. For intelligible extension does none of the work of a Cartesian idea. As a result, I argue that, taken to its logical conclusion, Malebranche’s position abjures the whole picture of the mind using a representation to think about the material world.

Whether each step of this debate represents a historical accident or a reasoned progression toward Malebranche’s final position is, of course, among the questions I hope to answer. By working through that sequence of positions and arguments, I hope to show, at a minimum, just how formidable are the challenges created by the twin demise of the Aristotelian and Baconian views, and just what lengths our philosophers are willing to go to in order to solve them.

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