Descartes now turns back to his task of inventorying the mind for clear and distinct perceptions. He has already told us what is clear and distinct in our perceptions of substances and what is clear and distinct in our perceptions of principal attributes. What is left now is to tell us what is clear and distinct in our perceptions of modes.
Sensation gives us the ideas of many different properties in the world. It tells us that physical substances have color, taste, smell, sound, heat, cold, pain, pleasure, size, shape, number, and motion, just to name a few. But not all of these, Descartes warns us, are really modes of physical substances. Color, taste, smell, sound, heat, cold, pain, and pleasure (which are commonly known as "secondary qualities") do not belong to physical substances at all. Though we think that these properties exist out in the world, they are really only in our own mind.
Descartes' proof for this claim rests on the relationship he posited between a mode and a principal attribute. A mode is just a determinate way of being a certain principal attribute. The principal attribute of body is extension. Any mode of body, then, must be a determinate way of being extended. Being square is a determinate way of being extended. Being two feet by two feet by three feet is another way of being extended. Being red, however, is not, nor is being sweet or painful. No matter how you manipulate the properties of extension, you cannot get out redness, sweetness, or the sensation of pain.
When talking about this class of qualities (i.e. the secondary qualities as opposed to the primary qualities) we must, therefore, be very careful if we are to avoid error. We must always bear in mind that these sensations do not clearly represent properties of bodies.
The discussion of primary and secondary qualities concludes the inventory of the mind. We are now armed with a method for attaining certain knowledge in all subjects, as well as all the clear and distinct metaphysical ideas that we will need to draw upon in this search for knowledge. We know that God exists and is perfect; we know that the essence of mind is thought; we know that the essence of body is extension; we know that mind and body are really distinct; and, finally, we know that bodies are not colored, smelly, tasty, noisy, cold, hot etc., but that they are shaped, sized, numbered, and motive. We are ready, in other words, to move on to Book II, the principles of physics.
Descartes' metaphysical positions have not withstood the test of time very well. Very few people believe that mind and body are distinct. Even fewer would ever use the concept of a "substance" or an "essence" in trying to describe the contents of the world. One of Descartes' positions, however, has been validated by all subsequent scientific discoveries and remains a part of our modern conceptual apparatus: the distinction between primary qualities and secondary qualities. Advances in chemistry and physics have shown that we do, in fact, live in a colorless, odorless, tasteless world—that these properties only come into our picture because of our own physiology. Philosophers, reflecting on this fact, have done a lot of work in trying to pinpoint what exactly this means for secondary qualities. If secondary qualities only come into the world because of us, does that mean that they only exist in our minds? Or does it, rather, mean that they exist in the world but as an arrangement of atoms with the power to cause certain sensations in us? Or do secondary qualities exist as relations between the arrangement of primary qualities of objects and our neurophysiology, relations that enable us to have sensations of secondary qualities? The three views expressed in the form of these three questions are referred to as "sensationalism," "physicalism," and "dispositionalism," respectively. Given the importance that this issue has developed, it is interesting to ask which of these three views Descartes himself held.
For a (still controversial) view on the history of western mind body dualism see:
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