Logically, truth is a property of sentences. For example, “snow is white” is true if and only if snow is white. All this requires is that we mean the same things by the words we speak and that snow actually is white when we look at it.
But what about perceptual illusions? If a stick dipped in water looks bent, is it true that it is bent?
No, it is true that is looks bent, but when we touch it at the seemingly bent spot or pull it out of the water, we find that, in fact, the stick is not bent.
And what about inductive reasoning? The sun will probably rise tomorrow morning, but how can we be certain that the statement “the sun rises every morning” is true? Certainly, we cannot deduce it from metaphysical principles.
Well, we know from all our personal and collective past experiences as well as from our astronomical understanding of the solar system that, from an earthling’s perspective, the sun rises every morning.
But how can empirical evidence be the touchstone of truth if our scientific understanding of the world is constantly changing?
Ultimately, truth is a matter of Bayesian updating: given the evidence we have, we can estimate the probability of a statement being true, and whenever we find new evidence, we update our truth estimate along with the new prior. This entails that truth is not static and reveals the primary epistemological principle, namely, that uncertainty is never vanquished, but only ever managed—managed, however, with the mathematical rigor of probability theory and variational methods.
In this stochastic sense, the truth of a sentence, model, or theory can be measured by the degree of accuracy and reliability of the predictions it entails.
Finally, what about people who want absolute truth—the Truth—which is never-changing, never-ending, eternal?
Unless these people are mathematicians,1 what they really want is to stop thinking about the world. Yet we shall resist their intellectual laziness and religious attitude. We do not want our curiosity to desiccate! For we know that it is among the greatest sources of our joy and success.
- Note that there are two types of truth: (1) the truth of synthetic propositions based on empirical correspondence with the world (the map reflecting the territory) and (2) the truth of analytic propositions based on logical coherence (the math following from axioms or natural-language sentences that are true by definition).