The answer is that some other recollection concurs with, and adds its quota to the support of, the one in question. When, in the view of one natural prospect, we recall another with great fulness, the present sensation supplies or fills in the parts of the remembered scene; which scene, therefore, does not exist in the mind by memory alone, but as a compound of memory and actuality. So while listening with pleasure to a band of music, we remember strongly the pleasure of some previous musical performance; yet, the vivid consciousness of the past is not dependent upon the memory of the past, but upon the stimulus of the present; we are more properly under sensation, than under idea. In all mental resuscitation, there is a degree of vividness and of fulness, due to the proper retentiveness of the mind for each particular thing, according to natural power, repetition, &c. Whatever is beyond this, must be ascribed to the accidental concurrence of other stimulants, either of present sensation, or of remembered impressions. {65} In recollection, there is an influence designated by the term "excitement," which means that portions of the brain are in a state of exalted activity. Any ideas embodied in the parts so excited, if in operation at all, are more than ordinarily vivid. Thus in fever, faded memories brighten up into vivacity and clearness. To this case the same remark applies; the result is partly memory, or the proper retentiveness of the system, and partly an excitation of the brain, through present influences. The proper power of memory is a constant quantity, varying only with repetition, and the strict conditions of memory; the intensity or fulness of a resuscitated idea is a complex result of memory proper and present stimulants, or sensations. Difference of vividness was the only distinction adverted to by Hume in his Psychology, which resolved all our intellectual elements into Impressions and Ideas. His opening words are:--"All the perceptions of the human mind resolve themselves into two distinct kinds, which I shall call _impressions_ and _ideas_. The difference between these consists in the degrees of force and liveliness, with which they strike upon the mind, and make their way into our thought or consciousness." He afterwards allows that in particular circumstances, as in sleep, in fever, or in madness, our ideas may approach in vividness to our sensations. Another distinction between the Sensation and the Idea, is of the most vital importance. To the Sensation belongs Objective Reality; the Idea is purely Subjective. This distinction lies at the root of the question of an External World; but on every view of that question, objectivity is connected with the Sensation; in contrast to which the Idea is an element exclusively mental or subjective. _Meanings of Sensation_.--The word Sensation has several meanings, not always clearly distinguished, and causing serious embroilments in philosophical controversy.