A Hybrid Approach to Should and Must

This paper takes a hybrid approach, combining the time and causality frame approaches to examine the interchangeability between should and must, which are epistemic modal auxiliary verbs that indicate logical inference and deduction. It is argued that only must is used when deduction is temporally situated at the present or in the past and directed from consequences to causes, but only should is used when the same process is situated in the future and directed from causes to consequences. The paper also shows either should or must are used in the remaining two types of cases—that is, those in which the process of deduction regards future situations and moves from effects to causes, and those in which deduction regards present or past situations and moves from causes to effects.