In René Descartes’ Meditations, he not only gives arguments for the existence of the mind and God, but also provides a systematic method through which anyone can exercise natural reason and discover truth in any realm of knowledge, be it science, mathematics, or religion. This is the foundation of scientia, or demonstrable knowledge.

Descartes begins by rejecting all of his previous opinions, starting anew that he may discover with certainty what is truth, and what are assumptions or falsehoods. He also thus appeals to the Skeptics, pointing out that if he denies any knowledge can be certain as they do, and finds a contradiction in this hypothesis, he must be correct. Because they often mislead, Descartes rejects the senses as a reliable source of truth and as an indicator of an object’s existence. For example, one may dream similar experiences to what the senses communicate is reality when one is awake. When we are in a dream, we are not usually aware we are in a dream, so our circumstances seem just as real as when we are awake, when in fact they are not. In fact, is there any way we could determine that we are not dreaming right now? The truths we appear to perceive while dreaming, however, are mere contingent truths that could change to falsehood upon waking. There are still some necessary truths which remain the same both dreaming and waking, and it is these that Descartes shall attempt to uncover. Descartes’ second dilemma is the idea that a powerful evil demon could be deceiving us into believing what we see is real, when in fact it is not. He thus resolves to believe that everything in the world around him could be an illusion.

Next, using this hypothesis, Descartes ponders the nature of his mind. If all is an illusion, than he must be deceived by someone, be it God or an evil demon. However, in order for that being to deceive him, there must be something to deceive, and so he concludes that his consciousness must exist. He looks at all the attributes of the body and so far cannot conclude through them that his body exists, for in all its actions he may be deceived. Even if he is tricked into believing he is thinking, however, he still must think something in order to be deceived. Because he thinks, he (or his mind at least) must exist. Although he seems to perceive light, heat, etc., he cannot prove that they actually exist, or that the medium through which he experiences them exists -- only his mind. Although it seems as though corporeal objects are more easily perceived that thoughts, they must be judged upon to truly be perceived, and thus it is the idea of the thing in one’s mind that is truly perceivable.

What else can be proven using Descartes’ systematic method, if we may not be certain of corporeal things? We still cannot prove that we are not being deceived, unless we can prove either that it is not possible for anyone able to deceive us to exist, or prove that God exists and is not a deceiver. Since thoughts are the only things we have determined to exist, we must study those. Because some of his impulses are not in accordance with his will, we must conclude that some thoughts come from external sources. Also, as the idea of a thing and the actual object may vary widely (as in the idea of the sun as huge star, and the idea of the sun as a round, bright, object an inch across), one’s ideas may often be false. Only if an idea exists outside one’s mind can it be certain, and not merely one's imagination.

One such idea is the idea of God. Descartes assumes that a cause must have at least as much reality in it as its effect. Something may not come into existence unless something real causes it to be so. He also dismisses the idea of infinite regress; that one thing can be the effect of something else, which is the effect of another thing, and so on to infinity. If he is not the cause of all ideas, then something infinitely perfect must be. Of all ideas except God, Descartes believes that he is capable of being their originator. Because he is a finite being, however, he could not have conceived the idea of an infinite God. He would have to have been perfect to come up with the idea of a perfect God; but as he considers God to be more perfect than he, God must have created him and not the other way around, for the cause must be more perfect than the effect. Also, if he were independent of any other being, (i.e., he created himself) he would be perfect and not lacking in anything, but as he is lacking in knowledge, doubts, and is deceived, he must have been created by something else. Because we have proven Descartes’ mind to be real, the idea of God must exist. Since God is the most perfect, and therefore most real thing there is (and therefore more real than Descartes’ thoughts), God must exist.

Descartes continues to explore all the objections to his proof on the existence of Deity. In order to be the first cause of something, a being must be able to create something out of nothing; as Descartes is unable to do so, he cannot be the first cause. Although his parents or something else may have directly caused Descartes to exist, something must have caused that cause to exist, and so on, back to a first cause, which is God. It is also not possible that several things came together to create him, for then he would not attribute to God the ultimate unity that he possesses. Finally, as this idea did not come from the senses or from Descartes’ mind, it must be innate, placed into the mind by the flawless Creator Himself, and Descartes formed into an imperfect image of God.

From the knowledge of the existence of God and the mind, how can we attain knowledge about other things? It is impossible for us to be deceived by God, for deceit comes of malice and weakness, imperfect qualities, neither possessed by a perfect God. As Descartes faculty of judgment is from God, it must be perfect, and thus will not lead him into error if he uses it correctly. Yet we still make mistakes. Perhaps, then, because we are midway between the state of perfection and the state of nothingness, so too is our sense of judgment good but imperfect, and thus occasionally leads us into error. However, as error is not necessarily analogous to nothingness, this cannot be the explanation. Perhaps our judgment is more perfect because we are capable of being deceived than if we were not. As we do not always understand God’s purposes, this is a distinct possibility.

Descartes next observes that errors depend on two things: understanding and will. The will, or freedom of choice, is infinite and perfect, allowing one to affirm or deny anything according to one’s desire. Understanding, on the other hand, is imperfect, demonstrated by the fact that it increases as we progress through this life. Neither by itself causes errors; it is when one does not restrain one’s infinite will to the limits of one’s finite understanding. In other words, one errs most often when one passes judgment on when one has incomplete understanding. Although Deity could have not allowed people to will when they did not have complete understanding, this would take away the freedom most characteristic of this existence, and although He also could have made our understanding perfect, doubtless it is not so for His own purposes, furthering the perfection of the universe. Thus, to avoid error, one simply needs to restrict passing judgment until one obtains sufficient understanding.

Now that Descartes has discovered how to avoid error, he questions whether we can know material objects exist. Certainly we can imagine them, and, as we used a similar argument to prove the existence of God, why not to prove the existence of corporeal objects? We cannot do this because the nature of ideas of material objects does not necessitate that they exist as the infinite and perfect nature of God dictates that He exists. Descartes then observes that things which he can clearly and distinctly perceive must be true, if he is unable to be deceived in areas where he has perfect understanding. Thus certain necessary truths which we can clearly and distinctly perceive can be assumed to be true, such as personal experiences and the laws of mathematics. Thus, even if we are dreaming, if we perceive something clearly and distinctly, it is true. Because the truth of clear and distinct perception is contingent upon the existence of a deceitless God, all truth depends on God, and through reason and gained experience, we can know truth. Here forms the Cartesian circle; we know that God exists through clear and distinct perception; and yet this perception requires that God exist.

Because we can prove the truth of our experiences, we also know, because we perceive them clearly and distinctly, that material objects exist. Even if the object does not exist, its idea exists along with the possibility that it could be created by our perfect God. Imagination, though it seems integral to perception, is really from the body, for although we can conceive of any possible thing, we are finite in what we can imagine. Because ideas originating from the senses are clearer than those from the imagination, objects seen by the senses could not have originated from the same place as imagined objects, and if they did not originate from one’s mind, then they must exist outside of one’s mind. However, we often perceive things at a distance with our senses, and then discover the same object is entirely different up close, or dream the same things that we perceive when we are awake. Because of this ambiguity, we can neither conclude all ideas from the senses exist, or that none of them exist.

Descartes’ principle of duality asserts that one’s essence is still only one’s mind, and therefore must exist separate from the body. One’s mind is intellect; one’s body is devoid of intellect, and thus the two must be separate. However, we cannot deny that the faculty of perception exists, but as it frequently is not under the control of the mind, they are still distinct. Either this faculty exists in the body, which is somehow connected and yet separate from the mind, or God sends perception. As he is not a deceiver, however, and the senses are often misleading, this is impossible. Therefore the mind and body are connected, allowing one to feel pain when one is hurt, and allowing one to receive sensory data. Because the senses often mislead, and our bodily natures sometimes send incorrect information to our minds, our natural knowledge is not complete, and thus is not unified.

While the body is matter and is divisible, the mind is immaterial and cannot be separated from itself. This is evidenced by how we think about the mind and the body. One’s consciousness cannot be separated into distinct parts; if one loses a part of one’s mind, one cannot survive. The body, on the other hand, is separated into members by function, and can still operate without several of those members. It is better for our body to mislead us on occasion, as in the case of dropsy, than for our body to mislead us all the time, and thus we may still believe in the perfection of God. Because our senses are more often right than wrong, we may infer that what we perceive by the senses is usually true, although we cannot prove that that will always be the case.

While we now have a method for distinguishing truth from error, because we often must make quick decisions, it is not a feasible method for everyday use. Through clear and distinct perception, and increasing understanding, however, one can most often discern truth.