“Quid est veritas?” Pilate asked. Famously, his interlocutor did not answer, perhaps because Pilate didn’t give Him the chance. Then Pilate may have been (understandably) addled because the Answer was standing there.
Anyway, Aristotle, under less pressure, had a go at a definition (one that Pilate almost certainly would have known). He said, “To say of what is that it is not, or of what is not that it is, is false, while to say of what is that it is, and of what is not that it is not, is true.”
That is lovely, understandable, and complete. It—the definition—is called realism, a pleasant and accurate label. Actually, it is called Aristotelian or ‘moderate’ realism to distinguish it between the hyper and over-literal realism of his pal, Plato. That difference makes no difference to us today.
There are other ideas of truth, all of them wrong, which follow two main roads: idealism (everything exists in your head, therefore your head doesn’t exist) and nominalism (what’s in your head doesn’t exist, therefore there is nobody there to think up and fret over nominalism). But we’ll pass these by today, too.
Yesterday, we agreed it was true that ‘all men are mortal’ and that ‘2 + 3 = 5’. It is the nature of men to die and for integers to behave undeviatingly according to certain rules. These are universal truths. There also exist contingent truths, which are propositions that accord with Aristotle’s definition conditionally. Unfortunately, there is only word in English for both, which means universal and contingent truths are often confused—which leads to hurt feelings.
To explain. Universal truths are those which begin with indisputable axioms and lead inexorably and necessarily to certain truths. For example, once we accept, without proof and based on no evidence save introspection, that “For all natural numbers x and y, if x = y, then y = x” and a couple of other similar sounding axioms, it is necessarily true that ‘2 + 3 = 5’. Because we don’t know why or how the axioms can be true—we just know that they are—we don’t know why or how ‘2 + 3 = 5’ is true, except in the weak sense that we say the equation is true because the axioms and intermediate theorems are. But we cannot say why it didn’t turn out that ‘2 + 3 = 7’ (don’t even think of arguing over the symbols).
Contingent truths are those propositions which follow from premises that might themselves not be universally true. For example, if we accept “All cats speak French & Whiskers is a cat” then it follows, i.e. it is contingently true, that “Whiskers speaks French”. Yet nobody but a cat lady would run into the street and claim Whiskers’s linguistic ability were universally true. That’s because the first premise is, according to other well known premises, false. Therefore, on that evidence, the conclusion, while contingently true, is universally false. True and false simultaneously, at least speaking loosely, and therefore something to fight over.
The “Laws” of science are all contingently true. Any one or even all of these Laws may be universally true, but we don’t (possibly yet) know it. If they were universally true, then they would all be in the same epistemic boat as mathematics and logic. We would start with introspection, decide what follows from beliefs we just know are true, and then build theorem upon theorem until we reached the Law of Gravity.
That’s almost how it works, but not quite. Inside the Law of Gravity are several fudge factors, “constants” of the universe which are derived via observation, i.e. which are not deduced from first principles. And (we read) there are one or two other dicey premises which are not entirely convincing. The Law of Gravity, which nobody doubts in practice, cannot be said to be universally true (no pun; nay, not even from me), even though it contingently is.
Because the Laws of science are only, or at least, contingently true the premises which accompany them may be argued over. It is not unscientific to do so. It is prudent. When physicists argue over gravitation, it is clear to everybody that the conclusion is accepted because it is observed to hold in most places, and so discussion centers on the premises which would make the Law hold in all places.
The situation is different in climate science, for example, where the conclusion itself is in doubt (rampant global warming will kill half the population by 2009—oops, I meant 2017), and where the premises are so beloved that they are Not To Be Questioned. The (suitably modified to keep current) doom conclusion is contingently true, but that does not make it truly true, i.e. universally true. Failing to understand that distinction is what leads the weak to shout “Denier!”