Should : What both is not
(certainly) so and is (probably) so if certain - normal or moral
conditions - are the case.
Note that 'should' is
normative in two possible senses, namely with reference to factual or
desirable conditions, and that in natural language the condition(s) are
usually tacit and presupposed. ('You should know what you should do, in
The logical import of 'should' is not easy to analyse, and if
done this is normally done in the context of modal logic.
There also is a subtle interplay with other similar verbs: would,
could, might, may,
will, can, shall. The common difficulty with these is that they involve
some - tacit or explicit - reference to what is not the case, is merely
possibly so, is not known to be the case, or is in the or a future, and
also involve some - tacit or explicit - reference to conditions of many
Additional complicating factors here are times, tenses, and
1. Non-modal analysis of "should": A - quite possibly
too simple but useful - non-modal analysis of "should", speaking about
human beings, is as follows:
What a person a should do (and should not do) generally depends,
directly or indirectly, on a number of norms of behavior and ends of a
where what one should do is what conforms to the norms of behavior or
supports the ends of the group, and generally
the non-doing of what one should do, if known to others of the group,
risks some social punishment or sanction.
And such a person a does do what a should do, by reference to these
ends and norms if and only if a when given a choice of options what to
do selects that option that best conforms to these ends and norms.