the principle is that which is naturally known, so in the process of
the rational appetite, i.e. the will, the principle needs to be that
which is naturally desired. Now this must needs be one: since nature
tends to one thing only. But the principle in the process of the
rational appetite is the last end. Therefore that to which the will
tends, as to its last end, is one.

The third reason is because, since voluntary actions receive their
species from the end, as stated above (A. 3), they must needs receive
their genus from the last end, which is common to them all: just as
natural things are placed in a genus according to a common form.
Since, then, all things that can be desired by the will, belong, as
such, to one genus, the last end must needs be one. And all the more
because in every genus there is one first principle; and the last end
has the nature of a first principle, as stated above. Now as the last
end of man, simply as man, is to the whole human race, so is the last
end of any individual man to that individual. Therefore, just as of
all men there is naturally one last end, so the will of an individual
man must be fixed on one last end.

Reply Obj. 1: All these several objects were considered as one
perfect good resulting therefrom, by those who placed in them the
last end.

Reply Obj. 2: Although it is possible to find several things which
are not in opposition to one another, yet it is contrary to a thing's
perfect good, that anything besides be required for that thing's
perfection.

Reply Obj. 3: The power of the will does not extend to making