Scottish philosopher David Hume — a skeptic’s skeptic, and not exactly a vaunted figure around here — is famous, in part, for his criticism of inductive reasoning. (Induction involves moving from a particular (or a series of particulars) to some general conclusion.) We tend, for instance, to use inductive reasoning when linking cause and effect. If I lift a ball and let go, the ball falls. The ball behaves this way every time I release it. As far as I know, every single human who hoists a ball into the air and drops it notices the same thing. The ball invariably plunges toward the earth. I conclude, therefore, that a causal relationship exists between my letting go and the ball’s descent.

But, according to Hume, my reasoning is faulty.* No matter how many times I observe one phenomenon following another, I can never be certain that the first causes the second. To do so — to achieve certainty — would require knowledge of the principles underlying that causal chain. But I have no such knowledge. I don’t know, and can’t know, whether there is a causal chain.