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truth, for something to believe in. Descartes’ strategy was to assume that Aristotle’s entire approach to nature, to reality, is wrong and then to assume the same for Aquinas, Plato, Duns Scotus, William of Occam, and all the revered writers. He ceremonially placed the Bible—from Adam and Eve to the Hebrew prophets to the resurrection of Jesus Christ—in the same dustbin. He continued slashing every such thought and idea until he came to a proposition that was impossible to deny. It was both a philosophical and a psychological undertaking, and to it he appended a “don’t try this at home” caution: “The single design to strip one’s self of all past beliefs is one that ought not to be taken by every one.”
    Maybe grand abstract writings needed to be dismissed in this way, but what about the things that are right in front of me? What about, as Descartes put it in
Meditations on the First Philosophy,
the simple fact “that I am in this place, seated by the fire, clothed in a winter dressing-gown, that I hold in my hands this piece of paper”? Even these things fell by the wayside. The senses can’t be trusted either. The senses deceive. I might be dreaming, or drugged, or deceived by a malicious deity. If we are being serious about this project, then sights and smells and tastes, no matter how self-evident, must also be doubted. Strictly speaking, I can’t even be sure of the reality of my own body.
    Which leaves what? At the end of this remorseless reduction there is only one thing that remains, one proposition that can’t be denied, one sound, as it were, in the universe, like the lonely ticking of a clock. It is the sound of the thinker’s own thoughts. For can I doubt that thoughts are occurring right now, including this one? No: it’s not logically possible. So, humble though it is, we can call this a ground: bedrock.
    In this way, Descartes became one of those rare figures in history who have given the world a sentence that is a touchstone. “I am the way, the truth, and the life” was such an utterance, standing on one side of Descartes and his era. On the other side we have “E=mc2.” As philosophers since have pointed out, “I think, therefore I am,” or “Je pense, donc je suis,” or “Cogito, ergo sum,” does not fully encompass what Descartes intended. Once the acid of his methodological doubt had eaten its way through everything else, what he was left with was not, technically, even an “I” but merely the realization that there was thinking going on. More correct than “I think, therefore I am” would be “Thinking is taking place, therefore there must be that which thinks.” But that hardly has the snap to make it a slogan fit for generations of T-shirts and cartoon panels.
    The irony is that in shifting the focus onto the individual human mind, which everyone agrees can be a pretty flimsy and wayward organ, Descartes had arrived at the closest thing to a certain basis for knowledge. If my own thoughts are the only indubitable ground I can stand on, apparently they aren’t so flimsy after all, at least not all the time. As an early follower of Descartes put it, “Doubt is the beginning of an undoubtable philosophy.” Therefore the mind and its “good sense”—that is to say, human reason—are the only basis for judging whether a thing is true. With the “cogito,” as philosophers abbreviate it, and with the theory of knowledge that arises from it, which Descartes outlined in the
Discourse on the Method
and later works, human reason supplanted received wisdom. Once Descartes had established the base, he and others could rebuild the edifice of knowledge. But it would be different from what it had been. Everything would be different.

    T HE MASSIVE G OTHIC CHURCH tower that dominates the skyline of Utrecht stands oddly alone in the middle of a