 To resolve the tension, Davidson noted that while the cause-law principle requires that there be strict covering laws, it doesn’t specify the vocabulary in which those laws must be formulated. If particular physical event p1 causes particular mental event m1, and there must be some strict law covering this interaction, but there is no strict law of the form ‘P1 → M1’, then there must be some other law, ‘?1 → ?2’, which covers the causal relation between p1 and m1. That is, m1 and p1 must instantiate properties suitable for inclusion in strict laws; but since we know that M1 is not a property of this kind, m1 must instantiate some other property. Davidson’s ingenious deduction at this point was that this property must be physical, since only the physical sciences hold out the promise of a closed system of strict laws (Davidson 1970, 223–24; on the notion of a closed system, see  5.1  and the supplement on  Causal Closure of the Physical in the Argument for Monism).  Therefore, every causally interacting mental event must be token-identical to some physical event—hence, monism  (5.1):