Trapped by their fears, Realists retreat screaming whenever faced with a Relativist idea. Simply can’t swallow it. And they retreat into exactly the same lack of intellectual discipline they accuse litcrit of. Conditioned by our cruel education system, that treats children as cattle, they refrain from any philosophy not geared towards providing “correct answers to the test”. To win the argument, then, you should proceed not with sound argumentation — that would only lead them into rationalization — but instead with masking relativism’s sour taste of personal responsibility.
When told that “there is no truth” they can’t avoid hearing instead “everything that you know is false”. To add offence to injury, left to themselves they will champion the arguments that lead to the inevitable conclusion that there is no truth, namely that: A) no theory is guaranteed to be universally valid, B) That every evaluation is filtered by culture, C) that confirmation proof is infinitely weaker than falsifying proof and so on. They don’t care about arguments, all they know of is their disgust.
So, how to avoid their gut reactions? Firs of all, talk about externality to subject. In their lack of intellectual sophistication, they can’t discern between “truth” and “externality”, so they’ll think you agree with their foolishness.
Then you stress that just because a thing is external to you, that is no reason to be a cretin about it or how it works or what is its nature. You say it is not enough to classify or behold the external world, that he needs to deal with this world and make experiments with it, and in the long run, to try to understand it.
Now that you used the 2 keywords in the same phrase, «understand» and «long run», you have to wait for it to sink in. Maybe make an unexpected detour into some unrelated subject, or maybe find a way to stress the keywords again. But give them time!
After they get tired of chanting some (veiled moralistic) fairy tale about science, you might try to lead them into admitting (or swinging it themselves!) that no scientific conquest whatsoever was achieved ex-nihilo, instead being based on some prior development, “shoulders of giants” style. From this you can suggest that, as Newton used Galileo and Einstein used Newton, it seems likely that we will use all of them to make something even better. Some Realist might see the trap there, and if so they will bask and try to find some detour, so you’ll be forced to do some reassuring baby-talk-like “everything will be all right” chitchat. But to tell the truth, I don´t believe many will see it, instead they will think you are almost convinced, since they tend to confuse “even better” with “truer”.
If things go smooth, you can try something along the lines of “and then our grandsons will use us, and so forth.” Using your most contrived “how-wrong-I-was” face, you lay the final trap: You say the problem is we can never naively use truth itself to validate our theories — Newton didn’t understand Einstein, and Galileo didn’t grok Newton. So we never really know that we are right, we can just know when we are wrong.
This should bring about a hysterical fit of Realist fanboyism, as he almost shouts “Popper”!
Wait till he stops giggling and sobbing, you can’t talk to someone in that state! When he recovers his composure, make an analogy between Popper and Descartes, in that both contributed more by not showing what is true, but instead by focusing on what is not true. Proceed to generalize it into a rule of thumb (or heuristic, if he’s got a taste for though words) that «good questions are more useful than good answers».
If he is not lost yet, you might have to improvise right now, as Realism tries everything to avoid the final, obvious connection. But, come on, we’re talking to a Realist here, how bright can he be? So, just deliver the blow: Regardless of truth exiting or not, science (or for that matter anyone with any intellectual maturity at all) won’t use truth as a criterion for anything: If you use a theory, you don’t do it because it is “true”, but because it is useful at your personal circumstance. Sometimes there are even conflicting ideas, and you must use subtler criteria than mere approximation.
Since all of this is absolute basic common sense, whenever you see someone talking about “truth”, you are close to an intellectual tar-pit, right next to dogma and circular explanations. Even if the “truth” in question is being relegated to being “in the horizon”, never to be touched or seen or used, like the invisible dragon in the garage, in practice it must still be avoided at all costs, for it blocks both scientific method and good-old common sense.
If you insist, you can keep the idea of truth for religious reasons, just like children believe in the fairy-tooth, but you must be very careful about not mixing truth with Philosophy: In Philosophy only the more elaborate notions of approximation, reliability, usefulness, cheapness — and others related — only them can be accepted.
It is not a matter of proving that truth is false, that it doesn’t exist, it is simply: Truth is a blunt idea, a naïve concept, and at the end of the day, an outdated tool. It does not work any more — our world is more complex that that.
In the end, Realism is just a special case of Relativism, just like Newtonian physics are a special case of Einsteinian physics. Whenever your assumptions are safe from questioning, you can talk about Truth like a simplification of nuances. That also happens to be whenever you are not dealing with any interesting idea. Then the concept of «Truth» might be useful, it might even be reliable in some cases. But it is just not up to date.
False is a much more simple issue than true, it is so much easier to be wrong than to be right — being right is at best a temporary thing.