Definition. Post hoc fallacy occurs when one makes a conclusive causal claim purely on the basis of where something resides in a sequence of events.
Problem. However, few if any can expound what makes post hoc fallacy a fallacy. On the contrary, average minds are content to presume that this is a logical truth and the final one. Accordingly, often, all too often humans will give a tautological answer void of scientific content. But this cannot be accepted here. Hence, we must part company here and venture beyond mediocrity to put forth a conjecture that may be confirmed or refuted by observation and experiment. To wit: why is the post hoc fallacy a fallacy?
Further intensification of problem. Commonly, the post hoc fallacy occurs when one concludes thus, 'A then B; therefore, A caused B.' But why is it the case that correlation is not causation? For various reasons that we will not fully expound just now for sake of brevity, the basic concern is the relationship of how variables relate to each other. For instance, the typical concern is that correlation is not causation and/or that one has confused cause with effect. We take this as axiomatic, as a brute fact, but do we ever stop to consider why an effect must come after a cause rather than the other way around?
Solution. It must be admitted that it took this thinker some time and plenty of contemplation to arrive at an answer. However, the answer can be found in light of Mr. Einstein's Theory of Relativity. The reason something in the past cannot be caused by something in the future is that nothing can travel faster than the speed of light; were mass able to outcompete light, situations may be permitted where an effect precedes a cause, but since this is not the case this explains why certain strands of post hoc fallacy are indeed fallacious. Implicatively, this seems to make backwards time travel an implausible proposition. On the other hand, what would need to happen for an effect to go before its cause? If we accept the first law of thermodynamics which holds that energy can be neither created nor destroyed, then for a human to go before the full chain of causation that manifested in his existence would require going to a state prior to the big bang singularity, at which point relativity breaks down and, ultimately, as Mr. Hawking has written, before time itself was born. Thus, on the basis of everything we know, to truly exist as an effect prior to all causality and not just the proximate cause may be utterly impossible or else some of our esteemed scientists have got it profoundly wrong. In short, if these variants of the post hoc fallacy could be demonstrated to be methodologically tenable, it seems that this can only be accomplished by falsifying some of Mr. Einstein's most notable ideas; otherwise, we can content ourselves by calling it a fallacy.