How we know things

Epistemology for modern Skeptics: III, Probability as the criterion of (likely) truth

Probabilism is the third pillar that allows us to articulate a modern version of ancient Skepticism, and live by it

Philosophy as a Way of Life
8 min readApr 7, 2022
alleged portrait of Thomas Bayes, Wikipedia

Being a skeptic means to maintain an attitude of open inquiry about the truth. As such, Skepticism is not really a philosophy of life, but rather a philosophical stance toward everything, including philosophies of life. Which suits me just fine, since I’m persuaded that Simone de Beauvoir was right when she wrote:

“It would be useful to explain how certain individuals are capable of getting results from that conscious venture into lunacy known as a ‘philosophical system,’ from which they derive the obsessional attitude which endows their tentative patterns with universal insight and applicability.” (The Prime of Life 178)

Ouch. Be that as it may, a modern skeptical approach is based on the refinement of three ideas that were already present in ancient Skeptics like the Greek Carneades and the Roman Cicero, two of which we have discussed in previous installments of this series: coherentism, the claim that a given belief is justified if it is part of a broader set, or system, of beliefs; fallibilism, the proposition that no belief can ever be justified in a way that does not admit the possibility that said belief may, in fact, be false; and probabilism, the idea that the only reasonable criterion to decide whether to accept a given proposition is our best estimate of the likelihood that said proposition is, in fact, true. It is to this third pillar of Skepticism that we now turn.

Carneades used the word pithanon to refer to such likelihood, meaning something like “persuasiveness.” Cicero translated it as probabilis, the root of the modern term probability. Neither philosopher, of course, had anything like the modern quantitative sense of probability in mind. Rather, they thought that it is rational to give provisional assent to whatever notion is backed by good arguments and relevant evidence. Always, of course, keeping firm in mind that we could be wrong, and that new arguments or evidence may (and should) change our opinion accordingly.



Philosophy as a Way of Life

by Massimo Pigliucci. Practical philosophy, science, pseudoscience & good reasoning. Complete index of articles at