The books which defined the way The West thinks now.The Very Squashed Version of Rene Descartes "Meditations on First Philosophy" :
Condensed and abridged to keep the substance, the style and the quotes, but ditching all that irritating verbiage.
I: Many of the things I used to be certain of, I now know to be nonsense. To find some firm foundation for science, I must try to establish what is absolutely true. So, I'll imagine that some evil genie is deceiving me about absolutely everything.The Full (but still Squashed) version is somewhat longer, but really does condense what is said in the "Meditations" rather better than I could have done. Or rather, what I did when I took "Traditional and Modern Philosophy" back in 1976.
II: I can't be sure of the things I know, but I can be sure that I know things. I think therefore I am.
III: All ideas have a cause. The cause must either be inside me, or something else. Infinity and perfection are not within me, so the idea of an infinite and perfect God must have come from something outside me, so God must exist.
IV: A perfect God would not cause the imperfection of deceit, so He is not deceiving me about the things of which I have clear and certain knowledge.
V: I am certain that I know material objects, inasmuch as I can define them by mathematics. This knowledge doesn't make things exist, but my knowledge of God makes me certain that they are something.
VI: I imagine that I have a body and that my knowledge comes from my senses. Using several senses together I can determine what is true. But we don't always have time for this, so we often make mistakes.
A useful link for Philosophy, which is something that everyone should study a little of.
Hat Tip : Eaglespeak.