One of the most famous definitions of knowledge is given though not ultimately supported by Socrates in Plato’s Theaetetus. It is that knowledge is justified, true belief. To have knowledge of something one must not only believe a proposition is so but know why it is so. Otherwise someone could be correct by coincidence.
To have adequate justification for a position be necessary seems reasonable, but what about the criteria of truth? After all, every piece of information created by every field of science is uncertain, and therefore, not necessarily true. Science must prove Scientific Realism which it can never do. Every piece of data about existence is perceived by the mind and nothing that is seen, not even the smallest sub-atomic particle of existence and knowing how it is connected to every other sub-atomic particle, can tell us whether this gives us metaphysical truth. So, either truth is not a requirement of knowledge, or science provides us with not knowledge – only things which appear reliable using induction.
If we remove truth from the definition, we still have the requirement of justification, so we still have to explain why we hold something is the case or likely is the case. Any claim of metaphysics or ethics has to be justified outside of mere appearances and any claim of the phenomenal world (this includes both science and history) has to have proper evidence and argumentation defending it.
Truth would then be that which follows from necessity, which would be logical and mathematical proofs. These are conjoined with empirical data and non-rational premises (often created by passions) such as the desire for happiness and particular states of being to make arguments about the way the world should be. These arguments are not justified, because they cannot ground the reliability of that which cannot be known. If we accepted these statements as justifiable then we could do the same with any if-then statement where the premises are assumed as long as the conclusion is logically valid, though not true or justified because we haven’t shown that the premises are true, therefore the conclusion has no certain truth or justified belief behind it. This is why science and history is justifiable but not certainly true but ethics in the domain of action is a type of faith. It can never be rationally justified nor known as truth that such should be or that such-and-such in the metaphysical sense is.
All we can justify is logical truths and claims within the world of appearances because of repetition in the case of science and reliable record in the case of history. Each domain being respected according to its own function. Logic being the domain of taking us from premises to conclusions and the faculties of perception and introspection telling us what is seen, heard and felt but never what is or what ought to be.
