While reading answers to the Were alchemist right?, I have come across the answer by @AndrewSteane. All in all he claims:
But the idea that it might be possible to transform one chemical element into another is not at all a stupid idea. It is a perfectly sound idea. However it turns out that it requires processes that change the atomic nucleus, and this requires either the use of radioactivity or else high-energy collisions. It cannot be done by chemical reactions. So no amount of heating stuff up in ordinary fires or pouring one liquid into another or adding ingredients of this or that is going to work.
Now, I am not a physicist, but in my completely unrelated field the question "if something is possible by applying this and that" is orders of magnitude harder than just "I have done this and that and produced desired something".
How do we know that
no amount of heating stuff up in ordinary fires or pouring one liquid into another or adding ingredients of this or that is going to
actually transmute one element into another?