Milk drinkers tend to become heroin addicts

Obviously this argument would not give us a reason to outlaw the drinking of milk. Yet it has a true premise, just as the first argument, and more importantly, it embodies exactly the same reasoning as the first argument. In particular, both arguments assume that if almost everyone who does X did Y beforehand, then having done Y makes them more likely to do X (or, those who do Y tend to become people who do X). As the second argument above illustrates, that is clearly wrong.

If you were given the first argument, you could show that it is a bad argument by presenting the second as an example of the same reasoning. That is what we mean by refuting an argument by means of a counterexample. The second example is a counter-example to the belief in this form of reasoning.

Now in this case we have been able to lay bare the mistaken assumption upon which the mistaken reasoning rested. This can often be very useful, and according to our general policy of making everything explicit, we should make the assumption explicit, rendering the two arguments thus:

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