So far as I know, criticism of Griinbaum's argument has always focused on (2). Michael Friedman ([1]), for example, sees no reason why we must adopt a causal theory of time or, for that matter, any other reductionist analysis of temporal relations. Even if (1) is true, he argues, it does not follow that there is no fact to the matter whether two events are simultaneous relative to a particular inertial observer. I am entirely sympathetic with Friedman's scepticism concerning (2). But even while avoiding debate over convention-alism and causal theories of time, one has grounds for rejecting Griinbaum's argument. On what seems to me a natural reading, assertion (1) is false. In a straight forward sense, the relative simultaneity relation of special relativity is uniquely definable from the causal connectibility relation. It is rather ironic. To the extent that one is committed to the" if" clause of (2) one