René Descartes
Discourse on Method
The confidence and introspection of this work are more dazzling
than any of its philosophic tenets.    Descartes insists on only
limiting himself to propositions that he can know with absolute
certainty, and he starts off with his famous
cogito ergo sum -- I
think, therefore I am.  Yet within a few paragraphs, he has
proven the existence of God and the soul. . . . and then is off to
explain the stars and the moon, the arteries and the heart, and
so much more.  And all in less than 25,000 words!  This is
moving far too fast for modern philosophy, which would devote
a tome to a punctuation mark.  But Descartes' passionate belief
in the power of reason to unravel such complexities continued
to inspire adherents for many generations.  And though the
most influential modern thinkers are far more pessimistic in
their assessments of our ability to find the truth of these
matters, it is far from clear whether we are better off after
imbibing their bitter skepticism.

The Donald Cress version,
which was my introduction
to this work, is still in print
(in its fourth edition) and
widely used.  But serious
students should consider
supplementing this with
the guide of a shrewd
philosopher -- and my
recommendations here
would be the works of
Anthony Kenny or
Bernard Williams.