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human
A reply to the 'fallacy of composition' objection to Mill's proof of utilitarianism?

The charge of a fallacy of composition against Mill's Utilitarianism is essentially an objection to his jump from 1) Each person ultimately desires their own happiness (a sort of hedonism) to 2) Everybody desires everybody's happiness, and so the good is a maximisation of the general happiness (not just your own happiness). This logical move is important to the theory as it argues why the moral action is that which maximises the general happiness (the principle of utility) not just the action that maximises your own happiness (A sort of ethical egoism).

My question is, this seems like quite an obvious/strong objection to Mill's 'proof' of utilitarianism, but I cannot think how he could justify this logical jump, as it seems clear to me to be a fallacy of composition (attributing some feature of the members of a collection to the collection itself). So, How do utilitarians reply to this objection?
gpt
I think Utilitarians would argue that they are not actually committing it.  Mill is not arguing that happiness is merely desirable, he is arguing that it is *worthy of desire*.  Namely, Happiness is Good - specifically Mill is claiming that Happiness is *the only* Good.

"[x] is desirable to me, therefore it is desirable to all," is only a fallacy of composition if we mean by desire "capable of being desired."  But Mill does not mean that.  He thinks Happiness is objectively Good.  To put it Platonically, it is The Good.

Sidgwick defends the move by explicitly claiming that "The good of any one individual is of no more importance, from the point of view...of the Universe, than the good of any other.''  He is following Bentham here who says "Each to count for one."

Mill is making the same move in different words.  Good is Good, not for *you* specifically.  So when you say:

> 2) Everybody desires everybody's happiness,

Mill is saying "Everybody *ought* to desire everybody's happiness."  Or, more carefully, "everybody ought to desire, on aggregate, a maximization of happiness in terms of quantity and quality."  

Why?  Because happiness is the only Good.