Everyone depends on senses for information and awareness. When we want to know what the world is like, we look around us, listen, taste, smell, touch. Even scientific experiments depend on the senses. We mix two chemicals and observe what results, or we let some ball bearings drop and observe how they behave. Descartes would like this heavy reliance on sensory observation to stop. He admits that for some knowledge the senses are required. For example, I could not know what books were on my desk if I did not use my eyes to check. However, he does not believe that we need sensory input when doing science. In fact, he is convinced that the senses only mislead us in scientific endeavors. Science, he feels, should proceed strictly by tracing logical connections between ideas of the intellect, not by observation. The senses do not even initially provide us with the ideas that we use in this reasoning. We are born with them already in our intellect.
Descartes, therefore, begins the Principles with two skeptical worries meant to undermine our faith in the senses. He points out, first, that our senses systematically mislead us. For example, when we view a straight stick through water, it looks bent; when we view things from a distance we tend to see them as much smaller than they are, or even as a different shape. Not only are the senses periodically unreliable, however, they are also constantly and stubbornly unbelievable. When we sleep we often have sensations indistinguishable from those that we have when we are awake. We admit that those dreaming sensations do not correspond to reality, so why are we any more certain of our waking sensations? How do we know that any particular sensation we have is not a dream? We cannot. Therefore, Descartes concludes, better not to rely on the sensations at all, at least not when you are after certain knowledge (such as in science).
In our quest for knowledge, Descartes' suggests that we rely on pure intellectual ideas. By taking stock of these ideas and then deducing their logical results, we can arrive at all possible human knowledge. However, we must be careful not to start the process of reasoning from any old ideas we find in the intellect. Judgments can only be certain when the ideas concerned are clear and distinct. Only when a perception is clear and distinct can we proceed from that perception to knowledge. Clear and distinct perceptions, therefore, are the single most important tool in Descartes' toolkit.
A clear and distinct perception is just a perception to which one cannot fail to assent. So long as you are entertaining the notion, you cannot doubt it without landing yourself in a logical incoherence. So, for example, the perception that two plus two equals four would count as a clear and distinct perception. So long as you are aware of the meaning of the terms involved, you cannot coherently doubt the truth of this claim. Descartes believes that the same phenomena holds true of propositions such as "nothing can exist and not exist at the same time," and the ever-popular "I think, therefore I am."
Though you cannot doubt clear and distinct perceptions as long as they are before your mind, once they fall out of your awareness, doubt can creep back in. If you have stopped entertaining the proposition that two plus two equals four, but only remember the conclusion you reached, then you might begin to doubt the legitimacy of the conclusion. You might wonder whether your reasoning was really as airtight as you had thought while it was taking place, perhaps whether some evil scientist was responsible for putting that thought in your mind. Obviously, as long as doubt can keep creeping back in whenever a clear and distinct perception falls out of awareness, clear and distinct perceptions are not going to be much help in the search for knowledge.
Descartes, therefore, appeals to God in order to guarantee the truth of clear and distinct perceptions, so that we can believe in them even after we have stopped entertaining them. God, he claims, created us and, thus, also our faculty of reason. He is, therefore, responsible for our clear and distinct perceptions. In addition to being our creator, God is also infinitely perfect. If our clear and distinct perceptions were not trustworthy, though, God would be far from perfect. He would be a deceiver, mean and malicious. An infinitely perfect God would never give us a faculty that presented perceptions as indubitably true when really they were false. Therefore, we can trust our clear and distinct perceptions. So long as we remember that a conclusion was reached through a perception that was clear and distinct (i.e. indubitable) while it was going on, we can be absolutely certain that the conclusion is true.
Perhaps the most important clear and distinct perception of all is the perception that extension is the essence of body. It is this perception that enables all of Descartes' physics.
To say that the essence of body is extension is not just to say that extension is the most important property of body. Rather, it is to say that body simply is extension. What it is to be a body, is to be an extended thing. So long as there is extension, there is body, and so long as there is body there is extension.
Extension is just dimension. Bodies have extension in three directions—length, breadth, and depth. To be a body, then, is simply to have length, breadth, and depth. Bodies obviously have more properties than just length, breadth, and depth, though. For instance, they have a certain shape. These other properties, however, are simply determinate ways of being extended (also called modes of extension). A body can be extended as a square, as a circle, as a dodecahedron, or as any other conceivable shape. Size, too, is just a determinate way of being extended. A body can be extended five feet by twelve feet by two inches, or thirty centimeters by thirty centimeters by thirty centimeters, etc.
It is clear enough to see how size and shape are just determinate ways of being extended, but bodies also seem to have some other sorts of properties, such as color, sound, taste, smell, heat, and cold. How can these be determinate modes of extension? The answer is that they are not, and for this reason they are also not really properties of bodies. All properties of bodies must be deducible from extension, and these properties have nothing to do with extension. These properties, therefore, do not actually belong to bodies, at least not in the way that we perceive them. (They might be said to exist in bodies insofar as they are only arrangements of the size, shape, and motion of particles with the power to create the sensation of these qualities in us. Alternatively, these qualities might just be said to exist in our own minds.)
By ridding body of all things but extension and its deducible properties, Descartes turns the study of physics into the study of geometry (the mathematics of extended bodies). The certainty of mathematics, therefore, can now be imported into the study of the natural world.
Since to be body is just to be extended, Descartes believes that there is no such thing as empty space. What we typically think of as empty space between objects—for instance, the foot of air between the bed and the floor—is actually just insensible body. That space between the bed and the floor has extension. It is one foot by five feet by six feet. Therefore, it is body.
The only difference between space and the objects we think of as body, is that space has no sensible qualities. We cannot see space, or smell it, or feel it. Other than that, though, it is no different from a car or a bug or a planet.
Since the entire world is filled with body (there is not empty space between bodies) Descartes calls the entire universe a plenum, meaning that it is filled. The idea that space is a plenum leads to many interesting conclusions, such as the vortex theory of planetary motion, the globular theory of light, and the important proof that terrestrial and celestial matter are exactly the same.
Motion is a crucial concept in the study of physics, and so it is important for Descartes to prove that motion is a mode of extension. If motion were not a way of being extended, then the entire study of physics could not really be deduced from the principles of geometry.
Descartes, therefore, denies the common understanding of motion as an action by which bodies transfer place. Motion, Descartes objects, is not anything outside of bodies themselves. Motion is just a function of the relative position of bodies. To be in motion, according to Descartes, is to transfer from one group of (contiguous) bodies (considered at rest) to another group of bodies.
The "contiguous" is important because it keeps motion from being entirely relative. Obviously, every body is transferring its position with regard to some things, and not with regard to other things, at every moment. If you are sitting still in your chair right now, you are not in motion relative to your chair or to the objects in your room, but you are in motion relative to other planets, since the earth itself in spinning. In order to allow us to say absolutely that something is at rest or in motion, Descartes adds in the "contiguous." The motion of a body, strictly speaking, is determined only in relation to those bodies with which it shares a common surface. When you are sitting still in your chair, then, you are not in motion, because you only share a common surface with the chair, not with heavenly bodies.
The "considered at rest" is important for a similar reason. Really, since motion is just the transfer of position relative to contiguous bodies, A cannot move away from B without B also moving away from A. They are both changing position relative to each other. This would all be well and good, if it were not for the fact that the Church did not want anyone claiming that the earth moved. Certainly, many contiguous bodies move relative to the earth (e.g. atmospheric particles). If a B must move in order for any A to move, the earth itself must move. Therefore, Descartes added in the "considered at rest." Though in reality B must move if A moves, when inquiring into the motion of A we consider B as not moving.
Given that the senses are supposed to be almost entirely left out of our search for knowledge, for what good then does Descartes thinks they are intended? Actually, Descartes thinks that they are very, very good at what they are meant to do, which is to provide us with information that enables us to get around in the world. The senses do not belong to the mind (responsible for our intellectual ideas) and they are not supposed to act as the handmaiden to the intellect, providing it with fodder for scientific reasoning. They also do not belong to the body (joined to the mind in some mysterious way to form a human being). Instead they belong to the composite of mind and body (the whole person), and they tell us what is beneficial and harmful to this composite.
Sensation works through a series of nerve channels, connecting various organs to the brain (the bodily seat of the mind). Our eyes, ears, nose, mouth, and skin are affected by tiny bodies in the air, in water, in other objects, and they send nerve signals to the brain, which result in sensations. Obviously the most mysterious part of this picture is how the stimulation of nerves in the bodily brain can possibly result in the production of sensations in the unbodily mind. Descartes is not able to give an adequate response to this puzzle of mind-body interaction.