“The dissatisfaction with life and the world, felt more or less in the present state of society and intellect by every discerning and highly conscientious mind, gave in his case a rather melancholy tinge to the character, very natural to those whose passive moral susceptibilities are more than proportioned to their active energies. For it must be said, that the strength of will of which his manner seemed to give such strong assurance, expended itself principally in manner. With great zeal for human improvement, a strong sense of duty and capacities and acquirements the extent of which is proved by the writings he has left, he hardly ever completed any intellectual task of magnitude. He had so high a standard of what ought to be done, so exaggerated a sense of deficiencies in his own performances, and was so unable to content himself with the amount of elaboration sufficient for the occasion and the purpose, that he not only spoilt much of his work for ordinary use by over-labouring it, but spent so much time and exertion in superfluous study and thought, that when his task ought to have been completed, he had generally worked himself into an illness, without having half finished what he undertook. From this mental infirmity (of which he is not the sole example among the accomplished and able men whom I have known), combined with liability to frequent attacks of disabling though not dangerous ill-health, he accomplished, through life, little in comparison with what he seemed capable of;”

Source: https://archive.org/details/autobiography01mill/page/74/mode/1up pp. 74-75