Being mortal, think of mortal things
Consider, for a moment, his own attempt to diagnose the crisis:
The experiences of this period [...] led me to adopt a theory of life, very unlike that on which I had before acted, and having much in common with what at that time I certainly had never heard of, the anti-self-consciousness theory of Carlyle. I never, indeed, wavered in the conviction that happiness is the test of all rules of conduct, and the end of life. But I now thought that this end was only to be attained by not making it the direct end. Those only are happy (I thought) who have their minds fixed on some object other than their own happiness; on the happiness of others, on the improvement of mankind, even on some art or pursuit, followed not as a means, but as itself an ideal end. Aiming thus at something else, they find happiness by the way.An interesting thought, this, about the paradox of egoism, but obviously irrelevant to Mill. The starting point of his nervous breakdown was that his "conception of his own happiness was entirely identified" with being "a reformer of the world". So, he already met the conditions of Carlyle's theory, aiming not at his own happiness, but at the happiness of others. Yet somehow he remains unhappy.
I suppose we should not expect self-knowledge from a man who could observe, without irony, that the first "small ray of light broke in upon [his] gloom" when he "accidentally" read the passage from Marmontel's Mémoires that "relates his father's death [...] and the sudden inspiration by which he, then a mere boy, felt and made them feel that he would be everything to them – would supply the place of all that they had lost." Writes Mill: "From this moment my burden grew lighter." I wonder why?
Despite his startling self-opacity, Mill does say things that help us understand his problem, and its solution. The key, I think, is in the following lines:
Is the echo of Aristotle here deliberate? It is in any case illuminating. Mill's description resonantly recalls the argument in Book X of the Nicomachean Ethics that, since the best good must be "final without qualification" – worthwhile for its own sake, and not for the sake of anything else – it must be contemplation rather than morally virtuous action.
In [Wordsworth's poems] I seemed to draw from a source of inward joy [...] which had no connexion with struggle or imperfection, but would be made richer by every improvement in the physical or social condition of mankind. From them I seemed to learn what would be the perennial sources of happiness, when all the greater evils of life shall have been removed. And I felt myself at once better and happier as I came under their influence. There have certainly been, even in our own age, greater poets than Wordsworth; but poetry of deeper and loftier feeling could not have done for me at that time what his did. I needed to be made to feel that there was real, permanent happiness in tranquil contemplation.
This argument has been found puzzling. On the face of it, "final without qualification" means "valuable but useless". (This is too quick, of course, but let me run with it.) It is a strange argument that attacks the moral virtues on the ground that they actually help people!
But what Aristotle says is not so strange; it is akin to Mill's remark about the need for good things that have no part of "struggle or imperfection". He says that "the virtues concerned with action have their activities in politics or war, and actions here seem to require trouble." In the case of virtuous action, or reforming the world, being worthwhile for the sake of something else goes along with a dependence on strife. It is because there is need, privation, hardship, vice, that the moral virtues must be active. But just as "someone would have to be a complete murderer if he made his friends his enemies so that there could be battles and killings" and thus occasions for courage, so it would be mad to want people to suffer so that one could be a saint. The ideal life would have no need for morally virtuous action. And so, if human life is not to be absurd, we have to find another picture of what it might contain. This is why we turn to what is "final without qualification", valuable but useless, like philosophical contemplation, or the poetry of nature.
I wish I could end on this note, but two problems remain. None of this explains why, when life is not ideal, we must still make room for the happiness of leisure, as both Mill and Aristotle claim. Is it simply so that we never lose sight of the point of fighting? And why should we concede that everything useful, everything worthwhile for the sake of something else, depends on needs we would be better off without? To put the point in its simplest form: what about work?