This made the first and most important problem the search for a single indubitable truth. Where can we find such a truth? Most of the knowledge about existing things that we normally deal with does not require indubitability, in most cases good approximations to the truth are sufficient. We can easily set these things aside. Senses have always been considered suspect. Different people perceive the same things differently and the same person can perceive the same thing differently at varying times and under differing conditions. So we can set aside those ideas which we gather directly from our senses. This, essentially, was Descartes' route to indubitability. Through systematic doubt he set aside any idea that was thought to be true but that was not clear and distinct, meaning that its rejection did not lead to any contradictions.

But this was not sufficient for the kind of absolute necessity that. Descartes was searching for. He took one more step beyond that, as we can see from this excerpt from his first meditation.

I shall then suppose, not that God who is supremely good and the fountain of truth, but some evil genius not less powerful than deceitful, has employed his whole energies in deceiving me; I shall consider that the heavens, the earth, colors, figures, sound, and all other external things are naught but the illusions and dreams of which this genius has availed himself in order to lay traps for my credulity; I shall consider myself as having no hands, no eyes, no flesh, no blood, nor any senses, yet falsely believing myself to possess all these things; I shall remain obstinately attached to this idea, and if by this means it is not in my power to arrive at the knowledge of any truth, I may at least do what is in my power [i.e. suspend judgment], and with firm purpose avoid giving credence to any false thing, or being imposed on by this arch deceiver, however powerful and deceptive he may be.

Indubitable truth is truth which can not possibly be false. Can such a truth be found entirely through the use of reason? Descartes' object through all of this systematic expulsion of both former beliefs and the sources of former beliefs was his application of the method discussed earlier. Remember he said that we must return to the simple, an uncomplicated, non complex truth which cannot be denied. All of the previous assumptions by which he lived his life he found were based on ideas that could have been false. Either he could have been mistaken about them or a malicious evil genius could have fooled him into believing them. So he went on.

I myself, am I not at least something? But I have already denied that I had senses and a body. Yet I hesitate, for what follows from that? Am I so dependent on body and senses that I cannot exist without them? But I was persuaded that there was nothing in all the world, that there was no heaven, no earth, no minds, nor any bodies; was I then likewise persuaded that I did not exist?

You should be able to see the problem here. How can he persuade himself that he does not exist, if he does not exist? Perhaps not, but what of the evil deceiver?

But there is some deceiver or other, very powerful and very cunning, who ever employs his ingenuity in deceiving me. Then without a doubt I exist also if he deceives me, and let him deceive me as much as he will, he can never cause me to be nothing so long as I think I am something.

The idea that it was possible for one to think and at the same time not exist, then even considering all of the conditions he has put before it, it would still lead to a manifest contradiction. This was not a new idea in Christian thought. Augustine said it a little differently, he said that if I think and am mistaken then I must exist. For him it was a direct refutation of the skeptics who claimed that one could know nothing for certain. Thinking implies that something that thinks exists because thinking is an act of something capable of thought. Aristotle said that the mind is made up of thoughts and comes into being the moment it begins to think. In Aristotelian terms, then, existence is necessarily predicated of anything that thinks simply on the basis that it is thinking. However, for Descartes a retreat beck to Aristotle and Augustine was not necessary because the very idea of non-existence of something that is thinking implies an unquestioned contradiction. His conclusion, then, is that single indubitable truth on which he was going to build a total science of truth. I think therefore I am, in Latin cogito ergo sum. However, the only truth determined by this is that he was a thing that thinks in Latin, res cogitans.

Through our senses we see, feel, smell our bodies and the world that surrounds us. But we have already determined that we have reason to doubt our senses since we may be sleeping, our senses may be distorted through illness or circumstances, or a malevolent demon may be fooling us concerning what it is that we think we sense. Our bodies and the world around us may not exist at all. We have not determined at this point any logical connection at all between us as things that think and the environment around us as things that exist. However, since we are things that think we cannot deny that we have thoughts, we only know that they may not be veridical. Therefore, if it is possible for us to know our bodies and the things that exist in our environment, that is know them without question, we cannot know them through the senses. We can only know them through understanding.

What is the relationship between thinking and experiencing? More importantly can that relationship be determined directly from the single fact that he exists because he is thinking. Again, one of his assumptions was that what one is thinking of cannot be wrong. A person cannot be wrong about what he is thinking of unless he attempts to maintain that it is equivalent to something existing outside the mind.

Descartes explained this through a meditation on a piece of bees wax. This wax, fresh from the hive, still had the color and odor of honey. It had a distinctive shape. One could knock on it and it produced a sound. Over time, as it set by the warm stove, it lost its odor then its color. The wax softened and lost its form and texture. Yet it still remained the same piece of wax that was originally taken from the hive. However, we do not know this by the use of our senses, we know it only through our understanding. Thus knowing about the wax has nothing to do with its sensible qualities. The point he is making is that the sensual qualities of anything may very well be misleading, but its existence is not. Thus a substance is what is. Aristotle said that a substance was something you could talk about, an individual thing of which other things could be predicated but could not be predicated to anything. For Aristotle substances were not first principles, we could know nothing about them other than what was generally predicated of them. Descartes' concept of substance is a new idea, adapted from Late scholastic thinkers like Suarez, but more platonic in the sense that it tended to be a universal concept of being, an idea that would have been rejected by Suarez. What could be predicated of Descartes' substance was existence itself. As we can see now, since he has determined through his systematic doubting that he exists as something that thinks, he too is a substance, something that has existence. But in this case he is a mind. There is a difference between a mind and a piece of wax. The wax is extended in space, it is corporeal. A mind, on the other hand, is not extended in space. It is incorporeal. Thus there are two kinds of substances, mind which is spiritual, not extended in space, and body, which is corporeal, extended in space.

But what of things that exist in the world? We do, therefore, have clear and distinct ideas of the existence of some things in the world. This does not imply that we would not be mistaken concerning what they were, only that we recognize that other things exist. Once stub your toe on a stone and the non-existence of the stone is a contradiction even if you did not see it. It is so even if you did not realize what it was. To know that something exists, that it is a substance, is to know only that and no more. It is not something that can be determined by the senses. It is something that can only be determined by the understanding.

The existence of anything implies that it had a cause, that it was created. What is created must have been created by something more perfect than it itself is. In order to prevent an infinite regress there must be a first cause and that first cause must be more perfect than anything that has been created. This, of course is God. The question this brings up is that if God, the infinite craftsman, made the things of the world and if these things are not perfect, then doesn't this imply an imperfection in the craftsman? However, if we return to the idea that God is both perfection and pure being, then we should be able to envision nothingness or non-being as pure imperfection or error. In a purely platonic sense, then everything that exists does so by participating in both existence measured as degrees of perfection and in nothingness, as measured by degrees of pure error. This is Descartes' answer to the problem of the imperfect world as a creation of a perfect God. This is a problem that set Spinoza and later Leibniz off on new explanations of God and existence. Thus degrees of perfection are measures both of the participation of individuals in the perfection of God and of our own clear and distinct ideas of their existence.

The problem of existence posed another problem for Descartes. Considering that he lived during a period when the general cultural view of the world was a kind of machine where everything that occurred did so through some kind of mechanical operation, his proposal that there are two kinds of substances, spirit or mind, and material or body, how is it possible that a spiritual soul can act on a material body? Descartes used the terms mind, soul, and self as essentially synonymous. This implies that passions of the mind that are caused by outside events as well as willful acts that result in bodily movements require some sort of mechanical connection between the spiritual soul and the material body. In his attempt to explain how this can occur, he began by making a distinction between functions attributed to the soul and those attributed to the body.

As to this we shall not find much difficulty if we realize that all that we experience as being within us, and that to observation may exist in wholly inanimate bodies, must be attributed to our body alone; and, on the other hand, that all which is in us and which we cannot in any way conceive as possibly pertaining to a body, must be attributed to our soul.

Descartes was impressed by Harvey who had recently discovered the circulation of the blood through the body. Descartes description of the inner operations of the body were done with due consideration to Harvey's work. As an addition to Harvey's description of the role of the heart and blood he added this description of the actions of muscles.

We further know that all the movements of the members depend on the muscles and these muscles are so mutually related one to another that when one is contracted it draws toward itself the part of the body to which it is attached, which causes the opposite muscle at the same time to become elongated; then if at another time it happens that this last contracts, it causes the former to become elongated and it draws back to itself the part to which they are attached. We know finally that all these movements of the muscles, as all the senses depend on nerves, which resemble small filaments or tubes, which all proceed from the brain, and thus contain like it a certain very subtle air or wind which is called the animal spirits.

It is these animal spirits that become the vehicle for communication between the spiritual soul and the material body. Descartes said that blood in the cavities of the brain form the animal spirits. "...what I here name spirits are nothing but material bodies and their one peculiarity is that they are bodies of extreme minuteness and they move very quickly like the particles of the flame which issues from a torch." Thus the reason some muscles contract while their opposites elongate is that these spirits issuing from the brain cause other animal spirits in the muscles to move very quickly from one muscle to the other.

When we sense objects in the external world then the senses themselves, being attached to the brain by the nerves, cause motions in the brain corresponding to the sensations. These motions themselves can cause motions in the animal spirits such that our muscles act without our willing them to do so. Thus it is possible for the members of the body to act in deliberate ways without the intercession of the soul. The result of this line of reasoning is that we can see that there is nothing that we need to attribute to the soul other than thoughts, which he claims are of two sorts, actions, and passions.

Those which I call its actions are our desires, because we find by experience that they proceed directly from our soul, and appear to depend on it alone: while, on the other hand, we may usually term one's passions all those kinds of perceptions or forms of knowledge which are found in us, because it is often not our soul which makes them what they are, and because it always receives them from the things which are represented by them.

This relationship between actions and passions causes a constant conflict between the soul and the body. The seat, or center of operation, of the soul, he said, is a small gland in the center of the brain he called the "pineal gland". This small gland is nudged around by the animal spirits from the outside which accounts for the passions, and in turn it pushes against the animal spirits which accounts for the actions of the soul. This may seem with our twentieth century background to be a bit strange. But, remember that this is the early seventeenth century. Forces which act other than by pushing were unheard of. They would be considered occult. This also results in a description of human psychology that emphasizes the new scientific concept of efficient rather than final causes being the important causes of events. The Greek way of thinking of the struggle of the soul as a struggle between the divine soul and the evil body through Descartes became a mechanical process. This Cartesian hybrid of religion and science became the basis for the new synthesis of philosophy and science that was to sweep the western world, even if only for a short period of time.