As we've seen, the opposite of certainty is doubt. Any doctrine that regards certainty as the normal condition of the human mind can be called dogmatism. Skepticism, by contrast, considers doubt the normal condition and even a logical necessity. Skepticism is suspicious of our faculties, where dogmatism considers them reliable. The former would have us remain in a state of equilibrium, adhering to no opinion whatever, where the latter would have us choose and commit ourselves.

Between these two extremes lies the doctrine of probabilism, which holds that probable truths – those that we can neither affirm nor negate completely but that can't be altogether doubted – exist. Practically speaking, we must have opinions about things. Those we adhere to are neither absolutely true nor absolutely false but simply have a greater likelihood than others of being true. Probabilism was the doctrine of the philosophers of the New Academy, including Arcesilaus and Carneades.

Let's begin by arguing against probabilism. The state of mind it deems appropriate – one of neither affirmation nor doubt – is unintelligible outside the context of certainty. For to say that one thing is more probable than another is to say that we are more certain of it. Remove certainty and all probability disappears. To say that one thing is more true than another, we must already have a criterion of truth. But if it's impossible for us to know what the truth is – if we can't be certain of it – then probabilism loses its reason for being.