This is why Newtonian physics was true in its day and age: there was no technology back then that exposed relativistic effects (nor could there have been such a technology, because no one had the know how).
This is why there is a grey area of claims that may seem to help predicting the future, but later turn out to be mere correlations rather than underlying truths. I.e. things that are "almost always true".
Newton in fact understood relativity very well, he just did not see the significance of it to the speed of light and time in our modern sense. Newton was not alone in understanding relativity, as you can go back as far as the sumerians and Dravidians and Egyptians and find the concept.
Surprisingly non of these topics are new to discussion, being regularly discussed in various cultures throughout history. Our contribution to the discussion has been technological and passionate. From the reformation the west really felt it could master nature and bend it to human will. Former cultures were more respectful and more in sync with their environment, calling each element or process a god and respecting it that way.
Relativity really required Lorentz to take off in Einstein's mind, but the fundamentals of relativity were appreciated solely through proportion, and trigonometry and Quaternions. Thus it took another 100 or so years to make this obvious to modern scientists, but many scientists from Maxwell onwards had a good enough perception of it. Einstein was the one who captured the scientific imagination and brought it all together, contributing very little math i might add! It was his insight and praxis that were influential. We then have to go through Schroedinger and Dirac and Feynman to get to the modern understanding. Now they did contribute some math!
As to truth, scratch that! Congruence, similarity and accuracy/ approximation are utilitarian cognates. Adhere,inhere and cohere are also important verbs to framework discussions.
There are no Absolutes! And that is absolutely true!

For the notion of moral relativism being a slippery slope, realise that all are hypocrites, and that takes care of that issue.

Empiricists have worked through these issues since greek times and before. John Locke is the modern founder of the movement.
There is nothing wrong with the old idea of "true" and nothing right with the old idea of "false", but this like any statement is to be taken with a pinch of salt.
Logic, particularly propositional logic is only utilitarian not "the truth" as pointed out above. In fact i hold that "the truth" is one of the major lies we are told in our current culture!
We may enjoy ourselves if we will accept that that is a great thing to do and an end in itself. Moral questions can then be seen as critiques to social mores as they indeed are. As a consequence of that mores are imperfect and prejudicial, unless we work to make them "fairer" in some way, and by some measure.
