Post hoc ergo propter hoc (Latin: ‘after this, therefore because of this’) is an informal fallacy that states: “Since event Y followed event X, event Y must have been caused by event X.” It is often shortened to “post hoc fallacy”.

The fallacy lies in a conclusion based solely on the order of events, rather than considering other factors potentially responsible for the result that might rule out the connection.

A simple example is “the rooster crows immediately before sunrise; therefore, the rooster causes the sun to rise.”

If people often commit post hoc fallacy, the question now, “Is it difficult to obtain causality?” Therefore we now ask, “What determines causality?”

You have to be careful. Not every two things vary simultaneously, then you can state it as “causality.” Maybe they are just “correlate” each other, not cause. Correlation does not imply causation.

John Rawls, a Harvard professor, said in his Theory of Justice, “Justice is the first virtue of social institutions, as truth is of systems of thought. A theory, however elegant and economical, must be rejected or revised if it is untrue; likewise, laws and institutions no matter how efficient and well-arranged must be reformed or abolished if they are unjust.”

As justice to a philosopher, truth is the greatest ambition of scientists. Searching and presenting truth is the ultimate goal of scientific inquiry. Science tries to give a conclusion that free from fallacy and inconsistency. This effort benefits you because science may not claim the final truth about anything, but it cleans your mind from fatal mistakes. It saves you time for finding the truth.