   These criticisms lead Reid to hold that there is a legitimate causal relation between two events whenever the two are conjoined by a law of nature, even if laws of nature do not simply amount to constant conjunctions. He uses the term physical causation to refer to the relation that holds between two events just in case they are conjoined by natural law. He takes the discovery of the physical causes of phenomena to be central both to the sciences and to ordinary life (see IHM 6.12, 122; EAP 1.5, 28–9; EAP 4.3, 211–2). But he also holds that genuine causation, what he calls efficient causation, is not reducible to physical causation. The reason is that, for him, a law of nature is not a brute conjunction between events. Rather, it is a regularity in the behavior of the efficient cause of observed phenomena. Consistent with remarks above (see 1.2 Newtonianism and Empiricism), Reid writes,