RETENTION. Whatever be the ef fect produced in the mental organs by the impressions on the organs of sense, that effect can be renewed, though in general with diminished vigour, without a repeti tion of the sensible impressions. In other words, sensible changes produce a ten dency to similar changes, which can be repeated without the repetition of the external impressions, and may then be called ideal changes. Less generally sen sations leave relicts behind them, which can be perceived without the agency of• the external organs of sensation, and which are called ideas. The power or capacity of the mind, by which tendencies to ideal changes are retained, may be call ed the retentive power.
That tendencies to a repetition of sen sorial changes are thus formed, that ideas are thus retained, may be referred to the operation of the associative power, and in the human being they certainly depend upon the same organic causes, whatever those be. But in many animals it is decid edly probable that sensations leave no re licts behind them; and in man there are, equally probably, numerous impressions from external objects, which leave no re licts behind them. Again, these relicts of sensations can re-appear without the agency of external objects. Hence it ap pears preferable to consider the receiv ing of sensations, and the retaining of ideas, as two separate, though intimately connected operations, and as implying two separate powers or capacities of the mind. This is not done by Hartley, who appears to refer both to sensation ; but it has subjected him to some apparentlyy, just, though in reality unfounded animad-, versions of the great northern philoso pher, Dugald Stewart. Speaking of the phenomena of memory as not to be en. tirely explained by the law of associa tion, he says, *(p. 412.) " The associa tion of ideas connects our various thoughts with each other so as to pre sent them to the mind in a certain or. der, but it presupposes a faculty of re taming the knowledge we acquire." This Hartley knew, and has according ly a distinct section on the generation of ideas.
Without the retentive power it is ob vious that man would be a being of mere sensation, little, if any, superior to the lowest orders of the animal cre ation, and inferior to many of them.
The retentive power provides materi als for the agency of the associative pow er. Without the retentive power the associative power would never be call ed into exercise, and without the associa tive power, the relicts of sensation, the effects of the retentive power would be of no utility. The operations of the retentive power can scarcely be sepa rated from those of the associative pow er, which together constitute the com pound faculty called memory, for an ac count of which see PHILOSOPHY, mental, ti 105.
We have said that the receiving of sen gations, and the retaining the relicts of them, seem to depend upon the same or. ganic causes, whatever they be. In some sensible changes perceptibly continue after the sensible objects are re moved. Two or three facts, which eve ry one must have noticed, or may notice, will illustrate this principle. If a piece of stick be burnt at one end, and the lighted end be turned quickly round in a circle, the luminous point will appear to the eye as a complete luminous circle ; the changes of the optic organs continu ing till the image of the luminous point returns to any given point of the retina. Again, the sensible changes produced by sound, perceptibly continue after the ex ternal cause ceases. If a sounding body be struck very rapidly with a stick, we do not perceive any interval, and as Hartley observes, the most simple sounds which we hear, being reflected from the neigh bouring bodies, consist of a number of sounds succeeding each other at different distances of time, according to the dis tances of the reflecting bodieS. The sen sible changes produced by the other senses, also continue some time after the impressions which have tbeen made up on them. if a hard body be pressed up on the palm of the hand, it is not easy to distinguish, for a few seconds, whether it remains or is removed. And tastes con tinue to be perceived long after the sapid material is withdrawn.