Has any noted philosopher ever offered a proof that "truth" exists?
No. In fact, they have often pointed out how we may never really be able to get it. Such philosophers range from Kant (with his idea that we can never know the "thing-in-itself"), to Hume (with his idea of "mitigated skepticism"), to Nietzsche ("there is no truth, only interpretation"), to the current crop of postmodernists.
But so what? As Richard Rorty, the American neo-pragmatist has said, "what is the use of truth?".
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How would we know once and for all that what we have is the ultimate truth? That would mean there could never be any other new observation, or new model, or new way of doing things, that would make us question it anymore. That sort of mindset is a surefire path to stagnation, closed-mindedness, and stagnation. We may be better off stopping talk of "truth", and talking about verification instead. It shifts the focus from the end result to the METHOD being used to make claims. That is how science works: no truth is ultimate in science. There are no sacred truths. The only thing sacred in science is the METHOD.
This neo-pragmatism mindset agrees with the postmodernists and Nietzsche that what we call truth is largely based on interpretation and can never escape our own current personal or community subjectivity. We can never get out from behind our own eyes and ears and see ultimate truth once and for all through what the philosopher Hilary Putnam calls a "God's-eye-view".
But following John Dewey's classical pragmatist mindset, these neopragmatists will argue that this is no license for complete nihilism or relativistic despair because some interpretations work much better in practice than others: they jibe well with the rest of our contingent web of beliefs, they empower us and allow us to do more, build more useful things, create more happiness and allow us to fulfill more of our potential as both individuals and societies, etc...
This is, after all, how science works, and it has been hugely successful. But no matter how good a model scientists have these days, and no matter how many mountains of evidence they have for a certain theory, they will blush if anyone calls their latest models and theories "ultimate truth". They always realize, rather humbly, that there can always be new observations, new more clever models, etc... which can come along and force them to rethink what they currently claim. This humility in science is not just a limitation, but rather ironically, is the foundation of its strength: because it always keeps their eyes and minds open to newer observations and models, and keeps them from sinking into closed-mindedness, and it is what keeps science so dynamic and quickly-growing.
In the next post, I will quote Richard Feynman, Nobel laureate in physics, on the importance of this sort of open-mindedness and humility. It seems to be the best compromise between the worst nihilism/despair of postmodernism and the narrow minded dogmatism of religion. But before that, I will quote the late Richard Rorty:
"As long as we try to project from the relative and conditioned to the absolute and unconditioned, we shall keep the pendulum swinging between dogmatism and skepticism. The only way to stop this increasingly tiresome pendulum swing is to change our conception of what philosophy is good for. But that is not something which will be accomplished by a few neat arguments. It will be accomplished, if it ever is, by a long, slow process of cultural change - that is to say, of change in common sense, changes in the intuitions available for being pumped up by philosophical arguments."
-Richard Rorty