NECESSITY, 293 saying that a prior event compels another to occur? And he cannot legitimately assume that he knows the meaning of this where the events are mental and not where they are physical. To the feeling of compulsion he might indeed have referred us, as an exclusively mental impression. But this he does not explicitly do. And the view that habits do compel the mind, not that we feel compelled by them, implies quite a different meaning of necessity, which he might just as easily have derived from the physical events themselves. This second meaning of necessity, which Hume thus seems to imply, is iu fact the very meaning that is involved in the connexion of cause and effect. We do commonly think that when some events have occurred others will necessarily follow ; and when we think this, we have no idea in our minds that we are compelled to think so. We do apply the idea of necessity directly to the connexion between two events ; and the only question is what is the idea that we thus apply. Hume certainly set out to answer this question, when he in- quired what impression it was of which the idea ' necessity ' was a copy. But in his answer he was led off into two quite different issues. His explanation is in the first place only an explanation of why we come to think it, not of what we think when we do think it. In order to get the latter, he would have had to introduce the feeling of compulsion : as it is, he merely assigns a cause for our belief that there are causes. And in the second place he confuses the question concerning the meaning of necessity with the question of its valid applica- tion to successions of events in time. He wishes to deny that there is any necessary connexion between events which are commonly called causes and effects : he holds that they are not necessarily related in the same sense in which two similar ideas are necessarily related. But this is to allow that necessity does mean something other than constant succession : for he does not deny that events have the rela- tion of constant succession. Hume has, then, certainly given no answer to the question : What is the meaning of that necessity which is commonly predicated of causes and effects ? In so far as he tries to explain why we come to think of certain events as necessarily connected, he seems to imply both that there is such an idea as necessary connexion, and that it may be validly applied to certain mental events. But, on the other hand, he holds explicitly that no- connexion except that of constant suc- cession may be validly applied to events ; and, in the second place, he points out a prima facie difference between two
