  Ontic necessities are those modalities universally quantifying over domains
of ontic possibilities, whose ``existence'' is independent of our knowledge. An
ontic necessity, called the weak ontic necessity, causes interesting questions.
An example for it is ``I should be dead by now''. A feature of this necessity
is whether it holds at a state has nothing to do with whether its prejacent
holds at the state. Is there a weak epistemic necessity expressed by
``should''? Is there a strong ontic necessity expressed by ``must''? How do we
make sense of the strong and weak ontic necessities formally? In this paper, we
do the following work. Firstly, we recognize strong/weak ontic/epistemic
necessities and give our general ideas about them. Secondly, we present a
complete logical theory for the strong and weak ontic necessities in branching
time. This theory is based on the following approach. The weak ontic necessity
quantifies over a domain of expected timelines, determined by the agent's
system of ontic rules. The strong ontic necessity quantifies over a domain of
accepted timelines, determined by undefeatable ontic rules.
