The huge difference is the scientific community did learn and moved on from alchemy. They didn't come up with some broad unprovable theory to justify it. You could argue religion was logical for the time but that is only true until contradictory evidence was discovered and religious people chose to ignore it. At that point it became completely illogical no matter what point in time it was. You would be 100% correct if religion had ended when science progressed and we were discussing it in a historical context but it didnt. To this day kids are still being taught the immaculate conception, that the earth is only a few thousand years old, that evolution is not real and all kinds of wild things that science has disproven. That's where he can chalk it up to mental gymnastics. The minute you create a theory out of thin air that cannot be proven or disproven to justify and continue your beliefs then you are no longer being logical in any time. Alchemists did learn. Religious people refuse to. That is the difference.