You are asking whether it is true that T=K (knowledge and truth are the same). From your asking this, I conclude that you don't know whether T=K. If truth and knowledge were the same, then lack of knowledge would be lack of truth. So, assuming T=K is true, we derive the conclusion that T=K is false. Better then to suppose that knowledge and truth are not the same. And of course they aren't. Something may be true and yet not be known by many or not be known by anyone at all. For example, take the following two sentences: "with optimal play, white can always win in chess" and "it is not the case that, with optimal play, white can always win in chess". One of these sentences is certainly true; but no one yet knows which one it is. (Or, if anyone does now, they haven't told me, so I don't know which is true.)

I think you are over-reading Oliver Leaman's point. He was contrasting philosophers to historians, pointing out that the latter can rely on a wealth of facts while the former cannot. So I believe he was referring to empirical facts ascertainable by observation, such as facts about bullets and bodies found underground beneath a Civil War battlefield. Such empirical facts constrain disagreement among historians (preventing historians from arguing that the battle was fought with swords and spears, for example). No such empirical facts constrain disagreements among philosophers - or so Leaman may plausibly be taken to have opined. This claim is less sweeping and fully compatible with your suggestions that there are truths in philosophy. This suggestion seems right to me. Some philosophical arguments are sound, others unsound. Some philosophical positions are coherent, others incoherent. Some philosophical objections are decisive, others fail. It would be quite natural to say that such truths express...

If you wanted to say something in favor of this view, you might point to the absence of observed discrepancies between what we all believe and the truth. But, on reflection, this isn't a strong argument because there are observed discrepancies between what is commonly believed now and what was commonly believed at some earlier time. At the earlier time, p was commonly believed. Now not-p is commonly believed. If what's commonly believed were true, then both p and not-p would be true. But p and not-p cannot both be true. Therefore it is not the case that whatever is commonly believed is true. Now you might say that what you mean is that truth is nothing more that what's commonly believed throughout the ages, the future included. So here is an argument against this revised view. There are lots of propositions about which there is no common belief shared throughout the ages: neither p nor not-p have been commonly believed. Does it follow that neither p nor not-p are true? For example, it has not been...