A note on Chapman's Modal Truth Criterion
The formal statement of the Modal Truth Criterion presented by Chapman  is shown not to be necessary, in contradiction of Chapman's original claim that the criterion is both necessary and sufficient. This is achieved through the presentation of a counter-example which is a partial plan in which a goal is necessarily true, and yet fails to satisfy Chapman's criterion. A corollary of this result is that the validity checking problem for partial plans, claimed by Chapman to be polynomially solvable, is now open.
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