What do we really mean when we say that a theory is "true"?

Jack says "The next train to London is at 11.15"; Jill adds "That's true".

Jill's remark in effect just repeats Jack's message. To say it is true that the next train to London is at 11.15 tells us no more about the world than that the next train to London is at 11.15.

Dora witnesses a crime. She gives quite a long statement. "Three boys in jeans and hooded tops came into the shop just before 12. They ... etc., etc. etc., ... And finally they jumped into a red car and sped off." Dick adds "That's all true." Again, Dick is in effect just repeating Dora's statement, but saving breath. You can see why we should have use for such a very handy device in our language. Someone says something, or we read something in a book; saying "that's true" has the effect of saying the same, without all the bother of repeating what is said or written.

And the same handy device is just as useful when what is said or what is written is not so common-or-garden but more theoretical. Alice says "The atomic weight of gold is 196.966". Ben says: "Hold on; I'll check -- [he Googles!] -- yep, that's true". When Ben says it is true that the atomic weight of gold is 196.966 he is just saying that the atomic weight of gold is 196.966.

Likewise, Charles produces a theory about why different people find different effects in holding a certain mobile phone in one corner, a theory that talks (let's suppose) about the different conductivity of different people's skin. Daisy goes off and does some tests, and comes back to report: she talks about the experiments, and concludes (let's suppose) "Charles's theory is true". Her concluding remark is just tantamount to repeating what Charles said in stating his theory, without the bother of saying it all again.

On this view, then, to say that a theory is true is, in effect, to say no more than would be said in repeating the theory.

More generally, what it takes for it to be true that P is no more than that P -- here fill in P by any proposition you care to pick. And that's the same whether P is very short and commonsensical (e.g. about train times), or is a long statement of theory (e.g. a textbook version of the theory of quantum tunnelling).

Of course, if P is long and theoretical it may be very hard to decide whether P is true! -- but that's not because 'truth' is mysterious but because it is difficult to decide whether P. It isn't that there are two tasks here, first deciding whether P, then deciding whether it is true that P: they are just the same task!

(The line I have been explaining is often called a "deflationary" or "minimalist" theory of truth. Of course the devil is in the details, and working out exactly how to present this sort of theory isn't easy. However, the various versions of deflationism/minimalism agree on this much -- the notion of truth functions in just the same non-metaphysical way whether we are talking about train times or quantum tunnelling, whether we are in the realms of common sense or high theory. Putting it crudely, in all cases, the notion of truth just serves as a handy device to enable us to say various things that would take too long without it.)

Peter Smith's use of the deflationary theory of truth to answer this question is just one way of approaching it. Another is to use the correspondence theory of truth.

According to (a highly simplified version of) the correspondence theory, truth is a relation between beliefs (or sentences, or propositions) and "reality": A set of beliefs (or sentences, or propositions) is true if and only if they correspond to reality, i.e., iff they "match" or accurately describe reality.

Now, a (scientific) theory is just a set of beliefs (or sentences, or propositions). So, a theory is true if and only if it corresponds to reality.

But how do we access "reality" so that we can determine if our theories (our beliefs) correspond to it? How can we do the "pattern matching" between our beliefs and reality? One answer is by sense perception (perhaps together with our beliefs about what we perceive). But sense perception is notoriously unreliable (think about optical illusions, for instance). And one of the issues in deciding whether our beliefs are true is deciding whether our perceptions are accurate (i.e., whether they match reality).

So we seem to be back to square one, which gives rise to yet another theory of truth: the coherence theory. According to (a highly simplified version of) the coherence theory of truth, a set of propositions (or beliefs, or claims) is true if and only if (1) they are mutually consistent, and (2) they are supported by, or consistent with, all available evidence--i.e., if and only if they "cohere" with each other and with all evidence. (Sometimes this is called a "pragmatic" theory of truth.) Note that observation statements (i.e., descriptions of what we observe in the world around us) are among the claims that must be mutually consistent, so this is not (necessarily) a "pie-in-the-sky" theory that doesn't have to relate to the way things really are.

Note that both Peter and I have been talking about "theories" of truth. So, which of these (or other) theories of truth is true? The answer to that is beyond our present scope! (But note that a correspondence theory must cohere.)

Perhaps it is worth taking continuing the conversation just a bit further.

The idea that a proposition (statement, belief) is true if and only if it "corresponds to reality" is -- as I'm sure William would agree -- not entirely transparent. What does it commit us to, exactly?

The deflationist about truth of course says that the proposition that snow is white is true if and only if reality is such that snow is white -- i.e. just if snow is white. So if the correspondence theorist is to be distinctively saying more than that, she needs to spell out what "correspondence" here comes to, over and above what the weak kind of correspondence that is already built into the deflationist view.

Now, there are indeed metaphysicians who do claim to have an "industrial strength" version of the correspondence theory, who postulate the existence of facts as ingredients of the world, facts which are truth-makers whose existence is required to make propositions true (where the worldly constituents of truth-makers are e.g. objects and universals).1 But it is far from clear that in saying something is true it is part of what we mean that there are such metaphysically weighty truth-makers as ingredients of the world.

For example, in our everyday truth-talk, outside the philosophy classroom, we are as ready to respond "that's true" to the claim that Angelina is beautiful or that slavery is wrong, as to the claim that snow is white. But are we really committed by this everyday way of talking to the existence of facts out there in the world which make true our aesthetic or moral judgments? Arguably not!

In other words, arguably not all everyday true propositions -- such as that Angelina is beautiful or that slavery is wrong (if you don't think they are true, choose your own examples) -- are made true by metaphysically heavy-weight facts, out there in the world. If that's right, then while the "industrial strength" correspondence theory might very well be part of the story about what makes some claims true, it isn't an account of the general meaning of 'true' across the board (indeed, there being or not being corresponding metaphysically weighty facts may be just what distinguishes some classes of truths from other classes).

As to the coherence theory as described by William, that sounds more like an account of how we tell (fallibly, no doubt) what is true, not an account of what it means to say that something, e.g. a theory, is true. Coherence of a theory with observation might well incline us, very reasonably, to think that it is true: but as skeptics love to point out, there seems to be no logical inconsistency in supposing that a theory is coherent but false.

1. Fine print: strictly this kind of truth-maker theory isn't a correspondence theory in the traditional sense, for it will deny that there is a one-one correspondence between truths and their truth-makers. For example, the same truth-maker, the fact that snow is white, makes true both "snow is white" and "either snow is white or grass is green"; and the fact that grass is green is another, different, truth-maker for the second proposition.

Read another response by Peter Smith, William Rapaport
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