Nuel Belnap

American logician who developed branching-time logic, the stit (sees to it that) theory of action, and four-valued logic for reasoning with incomplete or inconsistent information.

Stit theory: agentive sentences have the form 'agent sees to it that P' — a rigorous logical analysis of agency and responsibility. Branching time: time is tree-like with an open future; the actual future is one of many possible futures. Four-valued logic (with Dunn): True, False, Both, Neither — handles inconsistent databases without explosion.