The Meditations are generally considered the starting point of modern Western philosophy, and with good reason. In this one brief text, Descartes turns many Aristotelian doctrines upside down and frames many of the questions that are still being debated in philosophy today. Among other things, Descartes breaks down the Aristotelian notion that all knowledge comes via the senses and that mental states must in some way resemble what they are about. In so doing, he develops an entirely new conception of mind, matter, ideas, and a great deal else besides.
We might understand the philosophical outlook that Descartes develops to be marked and defined by the skepticism he employs in the First Meditation. He begins by asking how he can be certain of anything and then develops all sorts of inventive and outlandish reasons as to why he ought to mistrust his senses. Philosophy ever since has been marked a constant skepticism toward knowledge claims, and the very question of how we can come to know anything with certainty has been much debated.
Skepticism also informs the mind-body problem which has come to define our conception of the human mind. Descartes develops a conception of the mind where the senses and the imagination are also mental faculties. Further, he argues that we are essentially thinking things that can know our minds clearly and distinctly, but must work much harder to come to an understanding of our bodies. Most important, he draws a very sharp distinction between mind and body. Mind is essentially thinking and body is essentially extended, so the two have nothing at all in common. Ever since, philosophers have striven to understand how mind and body can interact and relate with one another.
Skepticism and mind-body dualism have combined to create an understanding of the human mind as being locked away inside a body and separated off from the world. How this mind can come to know anything at all about the world is a mystery, and the certainty of this knowledge is sharply questioned. This conception of mind is so natural to us that it is sometimes difficult to understand that the pre- Cartesian world had a far less skeptical outlook toward knowledge and sensory perception.
Descartes locates himself firmly in the rationalist camp, as opposed to the empiricism of Aristotle or his contemporary, John Locke. He constantly asserts that the clear and distinct perceptions of the intellect are the only sure means of securing knowledge, and ultimately concludes that the senses are not designed to give us knowledge at all, but are rather meant to help us move through the world in a very practical way.
While we can trace Descartes' tremendous importance and influence to the development of mind-body dualism and modern skepticism, he has also provided a number of other seeds for debate. The Cartesian Circle, the Wax Argument, and Descartes' theories of ideas, of body, and of perception are all important matters for discussion. His proofs for the existence of God, however, are not original, nor are they very successful. Descartes makes a fascinating subject for study since we can see a modern worldview emerging as he writes.