What, then, is the principle? We believe, he says,[475] that 'every event must have a cause,' and that circumstances exactly 'similar must have results exactly similar.' This belief, though applicable to all events, does not give us the 'slightest aid' to determining, independently of experience, any particular event. We observe that B follows A, but, for all we can say, it might as well follow any other letter of the alphabet. Yet we are entitled to say in general that it does uniformly follow some particular letter. The metaphor which describes cause and effect as a 'bond' tying A and B together is perfectly appropriate if taken to express the bare fact of sequence;[476] but we fall into error if we fancy there is really any bond whatever beside the events themselves.
The belief, then, in causation has precisely the same import according to Hume and Brown; and both agree that it is not produced by 'reasoning.' The proposition 'B has once succeeded A,' or 'has succeeded A a thousand times,' is entirely different from the proposition 'B will for ever succeed A.'[477] No process of logical inference can extract one from the other. Shall we, then, give up a belief in causation? The belief in any case exists as a fact. Hume explains it by custom or association. Brown argues, and I think with much force, that Hume's explanation is insufficient. Association may explain (if it does more than restate) the fact that one 'idea' calls up another idea, but such association may and often does occur without suggesting any belief. The belief, too, precedes the association. We begin by believing too much, not too little, and assume a necessary connection of many phenomena which we afterwards find to be independent. The true answer is therefore different. There are three sources of belief, 'perception,' 'reasoning,' and 'intuition.'[478] Now, we cannot 'perceive' anything but a present coincidence; neither can we establish a connection by any process of 'reasoning,' and therefore the belief must be an 'intuition.' This, accordingly, is Brown's conclusion. 'There are principles,' he says, 'independent of reasoning, in the mind which save it from the occasional follies of all our ratiocinations';[479] or rather, as he explains, which underlie all reasoning. The difference, then, between Hume and Brown (and, as Brown argues, between Hume and Reid's real doctrine) is not as to the import, but as to the origin, of the belief. It is an 'intuition' simply because it cannot be further analysed. It does not allow us to pass a single step beyond experience; it merely authorises us to interpret experience. We can discover any actual law of connection between phenomena only by observing that they occur in succession. We cannot get beyond or behind the facts--and therefore intuitionism in this sense is not opposed to empiricism, but a warrant for empirical conclusions. An 'intuition,' briefly, is an unanalysable belief. Brown asserts that a certain element of thought has not been explained, and assumes it to be therefore inexplicable or ultimate. Brown's account of causation had a great influence upon both the Mills, and especially affected the teaching of the younger Mill.