# Post Hoc Ergo Propter Hoc

post hoc, ergo propter hoc -- Latin for "after this, therefore because of this".

A LogicalFallacy in which we assume that because B follows A in time, A causes B.

Fallacious because:
• There might be a C which is a cause of both A and B.
• A and B could be coincidental.

A corruption of the logical argument "B follows A in time, therefore B does not cause A".

Related to CorrelationImpliesCausation (the fallacy where "A and B often go together, so A causes B").

Example:

95% of all crimes are committed within 24 hours of eating bread, therefore bread is a stepping-stone food to bigger crimes (moved from NaturalisticFallacy, because it's not).

However, this sort of idea *is* useful when casting about for a hypothesis, a reason why B occurs - it's perfectly logical to eliminate everything that happens *after* B, and cast suspicion on things that happened *before* B. Finding some A that happens before B lets you build a hypothesis "Perhaps A causes B".

The fallacy consists of deducing that "A causes B" from "B follows A", and from that fact alone. To show a causal relationship, you need a correlation plus a mechanism. Preferably a mechanism that can be tested to confirm or deny the hypothesis.

The crime-bread demonstrates another part that is artificial in logic. Assuming that if Event2 follows (in all known cases) Event1 it is caused by it. There is no proof possible as you cannot know the whole universe.

Huh? [Presumably saying common cause can't be discounted, a point already made.]

(EditHint: could we move all this to some page about monitors, magnetic fields, etc?)

I never had a CPU that cared which way it was facing, but I had a monitor that somehow became aligned to the Earth's magnetic field: When facing North, it was beautiful. When facing East or West, there was a dark spot on a part of the screen. We moved the monitor around; its relationship to the computer and furniture didn't seem to matter: only which compass point it was facing.

This makes sense. IIRC, the Earth's magnetic field is strong enough to deflect the electron beam in a CRT. This makes it a bear to get the geometry settings right after you move your monitor. I was told to degauss every 15 minutes the first few days after moving my monitor and then try setting the geometry.

Even repeated applications of a manual degaussing wand didn't help. Talk about an impediment to rearranging the furniture!

Alternate theory: Local magnetic fields caused by lights, electric wiring & cubical walls. Now, with two competing theories, we can perform tests to show which better explains the data.

(On monitor weirdness in general: Monitors can interfere with each other through cube walls. They can also be sensitive to fluctuations in electrical supply.)

Further: I have two monitors next to each other: a modern 17" flat tube and an ancient 14" fishglass tube. The old had bad colours and no degauss. Luckily degaussing the new monitor is powerful enough to degauss both screens. Or at least it makes the picture wobble on both screens the same.

...which reminds me of a trick in college, where the monitors where back to back. Degaussing your monitor would sometimes scare the bejesus out of the guy on the other side of the table. Childish, but fun nonetheless. -- AalbertTorsius

Perhaps something a bit more absurd?

The sun rises after the rooster crows, therefore the crow of the rooster causes the sun to rise.

-- GarryHamilton

Basically you run into a problem whenever there are multiple possible causes. Such as:

My friend died when he reached the age of 65, therefore becoming 65 kills you.

-- SvenNeumann

But in nature there is no because, no cause. In nature things just happen without any reason. The reason is just our way to describe the nature in simple terms. And the cause might follow the consequences. Example: The sun had to rise, just because some roosters wanted to crow.

The chicken died because I cut off its head.

In nature things just happen without any reason.

If that were true, science would be pointless, because nothing would be predictable. But science works, which means that effects do stem from causes. The computing technology we are all using is based on scientific principles. Your statement is disproved by the very medium through which you convey it.

from the weekly

the bill before the house "all crime is comitted by the chimney classes therfore the chimney itself is a criminal"

-- TheerasakPhotha

CategoryLogic

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