289. Having thus shown, as he conceives, what the true answer to the first of the above questions is not, Hume proceeds to show what it is by answering the second. ‘Since it is not from knowledge or any scientific reasoning that we derive the opinion of the necessity of a cause to every new production,’ it must be from experience; [1] and every general opinion derived from experience is merely the summary of a multitude of particular ones. Accordingly when it has been explained why we infer particular causes from particular effects (and vice versâ), the inference from every event to a cause will have explained itself. Now ‘all our arguments concerning causes and effects consist both of an impression of the memory or senses, and of the idea of that existence which produces the object of the impression or is produced by it. Here, therefore, we have three things to explain, viz. first, the original impression; secondly, the transition to the idea of the connected cause or effect; thirdly, the nature and qualities of that idea.’ [2]
[1] P. 383. [Book I, part III., sec. III.]
[2] P. 385. [Book I, part III., sec. V.]
a. The original impression from which the transition is made, and b. The transition to inferred idea
290. As to the original impression we must notice that there is a certain inconsistency with Hume’s previous usage of terms in speaking of an impression of memory at all. [1] This, however, will be excused when we reflect that according to him impression and idea only differ in liveliness, and that he is consistent in claiming for the ideas of memory, not indeed the maximum, but a high degree of vivacity, superior to that which belongs to ideas of imagination. All that can be said, then, of that ‘original impression,’ whether of the memory or senses, which is necessary to any ‘reasoning from cause or effect,’ is that it is highly vivacious. That the transition from it to the ‘idea of the connected cause or effect’ is not determined by reason, has already been settled. It could only be so determined, according to the received account of reason, if there were some agreement in respect of quantity or quality between the idea of cause and that of the effect, to be ascertained by the interposition of other ideas. [2] But when we examine any particular objects that we hold to be related as cause and effect, e.g. the sight of flame and the feeling of heat, we find no such agreement. What we do find is their ‘constant conjunction’ in experience, and ‘conjunction’ is equivalent to that ‘contiguity in time and place,’ which has already been pointed out as one of those ‘natural relations’ which act as ‘principles of union’ between ideas. [3] Because the impression of flame has always been found to be followed by the impression of heat, the idea of flame always suggests the idea of heat. It is simple custom then that determines the transition from the one to the other, or renders ‘necessary’ the connection between them. In order that the transition, however, may constitute an inference from cause to effect (or vice versâ), one of the two objects thus naturally related, but not both, must be presented as an impression. If both were impressions it would be a case of ‘sensation, not reasoning;’ if both were ideas, no belief would attend the transition. This brings us to the question as to the ‘nature and qualities’ of the inferred idea.
[1] Above, par. 195.
[2] Cf. Locke IV. 17, 2.
[3] Above, par. 206.
c. The qualities of this idea.
291. ‘’Tis evident that all reasonings from causes or effects terminate in conclusions concerning matter of fact, i.e. concerning the existence of objects or of their qualities’; [1] in other words, in belief. If this meant a new idea, an idea that we have not previously had, it would follow that inference could really carry us beyond sense, that there could be an idea not copied from any prior impression. But according to Hume it does not mean this. ‘The idea of existence is the very same with the idea of what we conceive to be existent;’ [2] and not only so, ‘the belief of existence joins no new ideas to those which compose the idea of the object. When I think of God, when I think of him as existent, and when I believe him to be existent, my idea of him neither increases nor diminishes’. [3] In what then lies the difference between incredulity and belief; between an ‘idea assented to,’ or an object believed to exist, and a fictitious object or idea from which we dissent? The answer is, ‘not in the parts or composition of the idea, but in the manner of conceiving it,’ which must be understood to mean the manner of ‘feeling’ it; and this difference is further explained to lie in ‘the superior force, or vivacity, or steadiness’ with which it is felt.’ [4] We are thus brought to the further question, how it is that this ‘superior vivacity’ belongs to the inferred idea when we ‘reason’ from cause to effect or from effect to cause. The answer here is that the ‘impression of the memory or senses,’ which in virtue of a ‘natural relation’ suggests the idea, also ‘communicates to it a share of its force or vivacity.’