If anything had changed with physical and chemical processes, we would find a completely different different set of properties between pre and post change. There are no differences in any physical or chemical properties of any substances on Earth or the observed universe.
Well, to be fair to Dad, our "uniformity of nature" assumption is somewhat foundational.
Indeed if, say, the rate of radioactive decay dramatically
and systemically changed there would be some indication (excess heating or even radiation-induced damage in crystals etc.), but if one posits that all of physics
could have hypothetically been different in the past in such a way that all things fall outside of our understanding of it, it would render our ability to draw reasonable conclusions into doubt.
This is precisely what Hume was running up against when he proposed Empiricism at it's extremes.
If the only way I know something, my only "epistemology" is via experiencing it, then I can't really draw conclusions on cause and effect. And indeed I cannot know that which I have not experienced.
The sun has always risen in the east and set in the west. I can only
assume it will do the same tomorrow as I have no way to "experience" the absolute causal foundation of that effect.
In the case of the "unobserved past" we have to rely on our "common sense" and an acquiescence that processes that occur today likely occured in the past at roughly the same rate and with roughly the same effects and responses.
What Dad wants to do is toss all that out. NOT to replace it with anything more robust but simply to decree the past "off limits" to any sort of understanding. Of course he will carve out an exception for the Bible. It comes from the distant past but it can be relied upon to give a perfect and accurate understanding of what occured.
I'm OK if Dad wants to drop the Epistemological Atomic Bomb on the discussion but I am
not OK with him acting like it doesn't affect
his own hypotheses about the past just because of some
ex cathedra declaration.