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.

本文讨论了本体必然性的不同形式以及弱本体必然性、强本体必然性、弱认识必然性和强认识必然性的逻辑形式，并提出了对这些必然性的完整逻辑理论，其中弱本体必然性涉及了预期时间线的可达性，在代理人的本体规则确定的预期时间线中，而强本体必然性则涉及到无法挑战的本体规则所确定的可接受时间线中。

分支时间中的强、弱实体必然性逻辑理论