"Some years ago I was struck by how many false things I had believed, and by how doubtful was the structure of beliefs that I had based on them. I realized that if I wanted to establish anything in the sciences that was stable and likely to last, I needed—just once in my life—to demolish everything completely and start again from the foundations."
These sentences begin Descartes's rational exercise of meditation. In them, he describes the background of this meditation and the primary motivation for his undertaking of the endeavor; they reveal an underlying skepticism in Descartes's method of thinking, and this set of meditations is the product of its exploration. This opening quote lays out the overview of the book: Descartes will logically reduce the world down to its mere essentials, if there are any at all, and then build it back up again from scratch in an attempt to discover what is true and what is illusory, especially in the realm of the senses.
"Even then, if he is deceiving me I undoubtedly exist: let him deceive me all he can, he will never bring it about that I am nothing while I think I am something. So after thoroughly thinking the matter through I conclude that this proposition, I am, I exist, must be true whenever I assert it or think it."
In this quote, appearing the Second Meditation, Descartes is debating within himself about the possibility of his own nonexistence. The skeptical side of his personality brings up the possibility that some sort of Supernatural Power is deluding him into believing that he exists. He responds by saying that if he can think about anything at all, including this possibility, then he must have some sort of existence, or else these thoughts would be existing in a void without a source mind, which is impossible. This quote also has a clear reference to Descartes's proposition of cogito ergo sum (I think, therefore I am) found in another of his works, Discourse on Method.
"So it seems reasonable to conclude that physics, astronomy, medicine, and all other sciences dealing with things that have complex structures are doubtful; while arithmetic, geometry and other studies of the simplest and most general things—whether they really exist in nature or not—contain something certain and indubitable. For whether I am awake or asleep, two plus three makes five, and a square has only four sides. It seems impossible to suspect that such obvious truths might be false."
This passage is intriguingly similar to Plato's philosophy on the nature and valuations of various types of sciences. Descartes aligns with Plato, who argues that true reality is to be found via the intellect beyond nature, far more than with Aristotle, whose theories hold that truth can be synthesized by observing the world through the senses. As Plato writes, mathematics is far truer than biological sciences, as they are dependent only on reason, and the principles are necessarily true without the need for external experimentation and verification. This hierarchy of sciences also gives the reader insight into what Descartes considers to be ultimately real.
