And lastly, even though the goal should be reached, the changes which
Time works in us have been left out of the reckoning: we forget that
the capacity whether for achievement or for enjoyment does not last a
whole lifetime. So we often toil for things which are no longer suited
to us when we attain them; and again, the years we spend in preparing
for some work, unconsciously rob us of the power for carrying it out.

How often it happens that a man is unable to enjoy the wealth which he
acquired at so much trouble and risk, and that the fruits of his
labor are reserved for others; or that he is incapable of filling the
position which he has won after so many years of toil and struggle.
Fortune has come too late for him; or, contrarily, he has come too
late for fortune,--when, for instance, he wants to achieve great
things, say, in art or literature: the popular taste has changed, it
may be; a new generation has grown up, which takes no interest in his
work; others have gone a shorter way and got the start of him. These
are the facts of life which Horace must have had in view, when he
lamented the uselessness of all advice:--

_quid eternis minorem
Consiliis animum fatigas?_[1]

[Footnote 1: Odes II. xi.]

The cause of this commonest of all follies is that optical illusion of
the mind from which everyone suffers, making life, at its beginning,
seem of long duration; and at its end, when one looks back over the
course of it, how short a time it seems! There is some advantage in
the illusion; but for it, no great work would ever be done.