Thus, for example, consider J.J.C. Smart’s (1959) thought that we should identify mental states with brain states, for otherwise those mental states would be “nomological danglers” which play no role in the explanation of behaviour. Or take David Lewis’s (1966) and David Armstrong’s (1968) arguments that, since mental states are picked out by their causal roles, and since we know that physical states play these roles, mental states must be identical with those physical states. Finally, consider Donald Davidson’s (1970) argument that, since the only laws governing behaviour are those connecting behaviour with physical antecedents, mental events can only be causes of behaviour if they are identical with those physical antecedents. At first sight, it might not be obvious that these arguments require the causal closure thesis. But a moment’s thought will show that none of these arguments would remain cogent if the closure thesis were not assumed, and it were thus left open that some physical effects (the movement of matter in arms, perhaps, or the firings of the motor neurones which instigate those movements) were not determined by prior physical causes at all, but by sui generis mental causes.