by Mark Kleiman

Sometimes people do damage to themselves, or incur the risk of doing so, at the (often not disinterested) prompting of others. Call this pattern "temptation," and the parties the tempted and the tempter.

The tempted, and their friends, often blame the tempters for the bad results, and sometimes sue them or demand that the activity of temptation be constrained by law. The tempters, and their friends, always respond that the tempted need to start taking personal responsibility for their actions, and that the tempters can't be held accountable for the foolish acts of others. (e.g., the tobacco companies, the alcoholic-beverage industries, crack dealers, gambling enterprises including both state-owned lotteries and privately-owned casinos, and the junk-food industries.)

The proposition that the blamelessness of the tempter follows from the responsibility of the tempted strikes me as obviously fallacious, but clearly many other people find it convincing. Since disagreeing with me is clear proof of error, the question arises as to the origin of this error, and I think I have found it.

The idea that "It's A's fault" logically implies "It's not B's fault" reflects the intuition that the moral responsibilities in a situation must sum to unity, like the probabilities of a set of events that partition an outcome space. But that intuition is transparently false. 

An outcome may be largely the result of chance, or of impersonal physical or social forces, in which case the human responsibilities sum to less than one, or it can be the result of independent actions by a variety of persons, each a necessary condition of the end result, in which case the sum of the responsibilities will be greater than one. When a teenager becomes obese by eating junk food, he, his parents, and the people who run McDonald's are all to blame; sharing the blame does not make it less.

After all, the Lord seems to have held Adam, Eve, and the Serpent all fully responsible for that unfortunate business with the apple.