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.