### A Logic for Weak Essence and Strong Accident

#### Abstract

Two new metaphysical notions, ‘weak essence’ and ‘strong accident’, are introduced. A proposition φ is weakly essential if once φ is true, φ is possibly true; and proposition φ is strongly accidental if φ is true but necessarily false. Under the condition that the frame is serial, if φ is essential, then φ is weakly essential, and if φ is strongly accidental, then φ is accidental. The relation between weak essence and strong accident, like that between essence and accident, is that one is the negation of the other. A logical system that describes weak essence and strong accident is established, and the system is shown to be sound and complete with respect to serial frames using possible world semantics.

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