On the face of the matter, it is absurd to ask whether it is more
important to know the limits of one's powers; or the ends for which they
ought to be exerted; or the conditions under which they must be exerted.
One may as well inquire which of the terms of a Rule of Three sum one
ought to know in order to get a trustworthy result. Practical life
is such a sum, in which your duty multiplied into your capacity,
and divided by your circumstances, gives you the fourth term in the
proportion, which is your deserts, with great accuracy.