A recent comment led me to one of my first ever posts, one about the Gateway Theory of drug use. For those unaware of the fallacy, it goes like this: many/most/almost all hard drug users started with softer drugs like marijuana, therefore marijuana causes harder drug use. It is the gateway to cocaine, heroin, methamphetamines, etc.

My post concluded with:

Let’s ignore for now the refutation that a higher percentage of cocaine and heroin addicts consumed alcohol than marijuana, and we all “know” that alcohol use does not cause cocaine or heroin addiction… (since many readers, like me, are occasional alcohol consumers who have never tried cocaine or heroin)

Let me ask you this: don’t you think the percentage of cocaine and heroin users that drank milk sometime in their lives (before use of the drug) is probably almost 100%?
Correlation does not prove causation. That’s just another logical fallacy brought to you by the Drug Czar.

Arthur left this comment:

This becomes pretty vacuous at the end. Do you win a lot of cases?

The reason one would think that "the percentage of cocaine and heroin users that drank milk sometime in their lives…is probably almost 100 percent" is unsurprisingly because the percentage of nearly everyone who drank milk sometime in the lives is probably almost 100 percent.

Yes, Arthur. That’s true, and therefore it would be inane to argue that because the use of [milk/marijuana] almost always precedes the use of [cocaine/heroin] that the first caused the second. Vacuous indeed. I couldn’t have said it better myself.