(Just so you know, I'm just kind of typing stuff as it comes to mind...)
I've been thinking about Descartes lately and problems I have with him. But then I thought about it more and realized that a lot of his problems are my problems too. Among the many things Descartes learned was the philosophy of Suarez. He probably studied his Disputationes Metaphysicae, where Suarez summarizes all the opinions available on basically every metaphysical topic there is. Descartes sees all of these opposed positions and thinks "Oh no, how can we know who's right?" He responds by taking a radical move--he rejects them all and decides to figure it out by himself. Now it's one thing to try to confirm what one is taught by critiquing the argument or by reducing it to first principles, but to reject being handed on anything from another teacher is problematic.
On the other hand, having picked up Suarez the other day, I now see why he was so intimidated. Whereas in math, you can follow the steps of a proof back to principles or in natural science, where you can (hopefully) repeat experiments that you hear about. It is very easy (in some true sense) to be certain about mathematical and physical matters. Now when one starts talking about the principles of things, it can be much more difficult to attain certitude. A comment or critique that is often thrown at philosophy is "Aren't you just arguing about words?" Unfortunately, this is many times the case! One is merely looking for logical coherency in a set of ideas (Kant?), but a true philosophy looks at things and tries to understand them by their causes.
Descartes not only rejects the teaching of others but he even rejects the evidence of the senses. That's definitely when he goes too far (I'll explain why in a minute...). But my purpose isn't just to point out why Descartes is wrong, but it is to be sympathetic with him. This isn't just Descartes being all, "Teehee, I'm going to destroy perennial philosophy on a whim, and cast the world into doubt and mistrust!" His reaction to Suarez (there are others; I don't want to blame Suarez, but it's a lot easier to say one name) and an over-convoluted philosophy (and I really can't claim to know much about the truth/false of Suarez or how it was presented--excuse the over-generalizations) was mistrust. It's like when someone you trust lies to you and now you don't trust anyone. Or perhaps closer, it is like having two parents who teach you opposite things about the most important matters--it causes a confusion that is difficult to reconcile. The mass of contradictions makes one seek refuge in what is closest--myself.
So Descartes' new starting place, Cogito ergo sum, is now a very lonely one. "Look at me, I'm doing philosophy all by myself with no help from anyone!" Kant too makes it his project to place the a priori principles of all things in his mind so that it is possible to be certain about all without relying on another. He realizes the consequence of what he's doing though: you can't have knowledge about any thing. Eek. Descartes does this thing where he proves God's existence and that God would deceive him, and therefore he an trust his senses and everything is back to normal. Kant realizes that if you start with yourself alone you can only end with yourself alone--that's why Kant makes sure you're mind is pre-programmed with a lot of cool stuff to keep you occupied in the mean time.
But the reason for all this is a desire for certitude! And who doesn't want certitude? I like certitude... The problem is that reality isn't always so certain, so you need to trust. "Oh no, but what if I trust the wrong person?" Yeah, that is a good question. That's why lying is always evil... The fact is that you need to trust someone and you have done so regardless! An infant is wholly dependent, receiving food, comfort, and basic education from others. Within this basic education is language, the tool by which we will communicate with all of those whom we may possibly trust and learn from.
Oh yeah, and the world! Kant eventually gets very particular about what "sense experience" we can trust (it ends up just being the "forms" of sensation, space and time). So what of this world? What of the purpose of philosophy itself which initially sought to understand the meaning of this world, of life, of anything at all? Yeah, it's problematic. Kant keeps it fun by making everything a priori, and I think we can probably learn a lot from what he did, especially about structure of thought and (by coincidence) of things. But it ends up being all form and no content! But we must be contentious. Not really. I was just running out of time and that pun came to mind. I apologize. I'll probably write about this more later, since philosophy, its purpose, our purpose, and so on have all been on my mind lately!
An exhortation: The best philosophy of all is about the best things! So for example, thinking about God is the best because he is the best. Be excellent! Do excellent things! Give people something good to wonder about! It is too often that people begin philosophy in times of doubt or of terror. Let wonder and joy be the beginning!