I am not the first to discover this particular fallacy—in fact, part of this post is based on ideas I got in conversion from somebody who got them from something he read. But the ideas are no less fun for being mainly not mine.
Consider the following argument for psychological hedonism, the doctrine that the only thing we pursue is pleasure:
- Whenever we pursue something other than pleasure, we pursue it because it gives us pleasure. (Premise)
- Therefore, what we really pursue is the pleasure it affords to us.
- We pursue x because it gives us y.
- Therefore, what we really pursue is y.
- Therefore, we pursue y
Perhaps the inference works better if we replace (1) by:
- Whenever we pursue something other than pleasure, we pursue it only because it gives us pleasure.
In fact, it is even incorrect to conclude from the claim that I seek x solely because it yields y that I want y at all. Suppose that, whimsically, I desire a magical hat that yields rabbits. I only want the hat because it yields rabbits—my whim is that I want to have rabbits pop into existence out of a hat. I can want such a hat for such a reason without having any desire for the rabbits. The rabbits themselves are a nuisance, and I would have no interest at all in rabbits that come into existence in any way other than out of a hat.
It might be objected that then I don't want the hat just because it yields rabbits, but I want the hat because it is a hat that yields rabbits, and so this isn't a counterexample to the inference type. But if so, then the non-hedonist need only say that she doesn't want x just because it yields pleasure, but she wants an x because it is an x that yields pleasure.
The fallacy here also occurs in the Lysis in the argument that if I am friends with x because x yields y, then what I am really friends with is y.