You began with the question of whether higher marginal tax rates made economic growth "impossible". After finding that there was growth between 1940 and 1980, y

You began with the question of whether higher marginal tax rates made economic growth "impossible". After finding that there was growth between 1940 and 1980, you should have concluded "no, it does not make it impossible as we have a counter example". This follows and is called induction. But you for some reason chose to then alter the premise and say you proved that high marginal tax rates are not harmful. You have in no way proved this. For all you know without those high marginal tax rates, growth would have been even HIGHER between 1940 and 1980. I am sure you do not believe this to be the case, and that is a very reasonable belief, but understand that it is not proven by this evidence (nor is it disproven mind you). True evidence-based proofs, the way its done with science, requires that you test one variable. In the example above you'd have to have a parallel economy where all else was the same except the tax rate. Unfortunately this is impossible, but this does not mean we get to change what "proof" means. Economics would work much better if everyone came to terms with the fact that it is untestable and was honest with themselves that all opinions regarding it are just beliefs, regardless which side you fall on.