  In this paper, we present a logic for conditional strong historical necessity
in branching time and apply it to analyze a nontheological version of
Lavenham's argument for future determinism. Strong historical necessity is
motivated from a linguistical perspective, and an example of it is ``If I had
not gotten away, I must have been dead''. The approach of the logic is as
follows. The agent accepts ontic rules concerning how the world evolves over
time. She takes some rules as indefeasible, which determine acceptable
timelines. When evaluating a sentence with conditional strong historical
necessity, we introduce its antecedent as an indefeasible ontic rule and then
check whether its consequent holds for all acceptable timelines. The argument
is not sound by the logic.
