Post Hoc Ergo Propter Hoc: This is a fancy Latin term that, loosely translated, means "after this, therefore, because of it." You commit this logical fallacy whenever you put together two facts or events, and imply that they are related simply because the one happened after, or around the same time as, the other.

I see this one used in political arguments all the time, and it frankly drives me batty. Probably because I used to commit this error all the time myself, and I want to kick myself for it.

Here is one classic example:

Jimmy Carter was President, and interest rates went up substantially. Conclusion: Jimmy Carter caused high interest rates.

This is a logical fallacy. Anyone who knows much about our monetary system knows that, at best, Presidents have a very tenous ability to control interest rates. They are mostly marginal players under all but the most unusual circumstances.

"Jimmy Carter was President, and interest rates were high. Therefore, Jimmy Carter caused high interest rates." This statement commits a logical fallacy. Period.

On the other side, less than three weeks after George W. Bush was sworn into office, our neighbor (a diehard Republican-basher) came into my house and laid this little gem on us:

Did you hear? Did you hear? Unemployment is going up! See, I told you, if you get a Republican President, the economy goes to hell

While one may argue about the long-term consequence of various policies the President may enact, or fail to enact, it is a logical fallacy to suggest that the presence of a Republican in the White House causes high unemployment in and of itself.

Scientists know this same logical rule in a more positively-stated form: correlation is not causation. For example:

My father had chest pains, and died. Therefore, he was killed by a heart attack.

The Earth is warming, and humans are emitting more Carbon Dioxide. Therefore, human CO2 emissions are causing the Earth to warm.

Both of these statements are based on a logical fallacy. Yes, in each case, you can correlate two facts in space and time: both coincide within a similar time frame, and relate to the same object (my father, the atmosphere). Each involves a logical correlation. But you cannot state that the one caused the other without more evidence: my father may have had chest pains for reasons completely unrelated to, or peripheral to, whatever killed him. Human CO2 emissions may be far too small to account for the global warming that some scientists think we're seeing.

I'm not splitting hairs, either. Ask any E.R. doctor or any pathologist about my "man with chest pains dies" scenario, and he'll back me up. Similarly, there are literally thousands of qualified scientists who feel so strongly that the "humans are causing catastrophic global warming" theory is reckless and foolish, they've actually signed a public petitition to that effect.

On the other hand, the man may have died of a heart attack, and humans might be causing catastrophic global warming. The point is that it is reckless to jump to conclusions based on a simple correlation.

This is not a technicality, either: it's a fundamental rule of science, and a fundamental rule of logic. Countless would-be scientists have been burned badly by failing to take this sort of rule seriously.

It is very convenient to simply assume that: because A happened, then B happened, then A caused B. It may also be irritating when someone points out that you can't make that assumption.

But if you do, you're committing a logical fallacy.

Post Hoc Ergo Propter Hoc, or "this, therefore, that." A much-too-common logical fallacy.