What are the mechanisms by which behavior is organized sequentially over time? The recently developed mid-session reversal (MSR) task offers new insights into this fundamental question. The typical MSR task is arranged to have a single reversed discrimination occurring in a consistent location within each session and across sessions. In this task, we examine the relevance of time, reinforcement...

• The Perfect introduces a time interval: the “perfect time span” (PTS). • The Right Boundary (RB) of the PTS is set by Tense. • The Left Boundary (LB) of the PTS may be set by “perfect adverbials”. • The lower predicate (event) is predicated of the PTS, either directly or mediated by various devices, most notably operators associated with the Perfective or Imperfective. • When the predicate ha...