Since, therefore, the whole discussion is to be on
the subject of duty, I should like at the outset to
define what duty is, as, to my surprise, Panaetius has
failed to do. For every systematic development of any
subject ought to begin with a definition, so that everyone may understand what the discussion is about.
Every treatise on duty has two parts: one, 1
dealing with the doctrine of the supreme good; the
other, with the practical rules by which daily life in all
its bearings may be regulated. The following questions are illustrative of the first part: whether all
duties are absolute; whether one duty is more important than another; and so on. But as regards
special duties for which positive rules are laid down,
though they are affected by the doctrine of the
supreme good, still the fact is not so obvious, because
they seem rather to look to the regulation of everyday [p. 11]
life; and it is these special duties that I propose
to treat at length in the following books.