This is what Descartes is getting at. He said, “Don’t just tell me what everybody else believes.
Give me the argument. Show me its validity.” He said, “Even my own
conjecture does not bring certitude. But I want to cut through all of the confusion
of my day and get down to those basic ideas, those basic truths
that are indisputable, that I can take to the bank, that I can live my life on—on
the basis of certitude.” And so, from this, he set out on what I call a “systematic doubt process,”
by which he, being from Missouri, that Southern part of France, he decided to question any thought
or any proposition that he ever heard—to question it to death. He questioned whether the hand that
he saw in front of his face was really there. He questioned his senses. He said, “How do I know
that what I am perceiving in the external world really exists out there, or is not simply
a hallucination in my own mind, or an illusion perpetrated by some wicked devil who’s fooling me?
I can’t trust, in the final analysis, with absolute certainty, what I perceive
in the external world. I’m not going to trust theological authorities. I'm not going to trust
scientific authorities. I’m going to doubt everything that I can conceivably doubt.”
So, Descartes embarked on this process where he doubted everything. But the one thing he couldn’t
doubt was that he was doubting. Because if you doubt that you’re doubting, what are you doing?
You’re doubting. So, one thing I know for sure is that I am doubting. If I doubt that I am doubting,
I prove that I am doubting. Because, to doubt doubt requires doubt.
So, he came to the certain conclusion that he was doubting. Then he said, “Okay, what else
is clearly contained within the idea of my doubting?” Well, for someone to doubt,
they must be thinking. Because doubt is a form of thinking. You can't have doubt without thought.
Because doubt is an element of thought. So now he says, “If I am doubting, then I know
that I am thinking. Now, if I am thinking, what else does that tell me through resistless logic,”
according to Descartes? “If I am thinking—I am having thought—there must be something
that is doing the thinking, because thought requires a thinker.
And it’s not your thought that’s giving rise to my doubts.
It’s my thought that I am immediately aware of through pure intuition, to know that I am the
one who is thinking and I am doubting. And if I am thinking, and I am doubting, I must be.”
So, he gives us his famous formula that we have all heard at one time or another,
pronounced variously by different people, and I am going to give you this pronunciation: “Cogito
Ergo Sum.” Now, perhaps the most important part of this formula is the middle word, “ergo,”
which means what? It means “Therefore.” It is the word which signals a conclusion, a rational
conclusion. He is saying, “Cogito”—"I think, I am cogitating”—therefore, “Sum”—“I am.” So,
he finally reaches something about which he can be absolutely certain: namely, his own existence.