Leaving the a priori conditions of sensation we may proceed to consider acts of sense. These admit of various degrees of duration or intensity; they admit also of a greater or less extension from one object, which is perceived directly, to many which are perceived indirectly or in a less degree, and to the various associations of the object which are latent in the mind. In general the greater the intension the less the extension of them. The simplest sensation implies some relation of objects to one another, some position in space, some relation to a previous or subsequent sensation. The acts of seeing and hearing may be almost unconscious and may pass away unnoted; they may also leave an impression behind them or power of recalling them. If, after seeing an object we shut our eyes, the object remains dimly seen in the same or about the same place, but with form and lineaments half filled up. This is the simplest act of memory. And as we cannot see one thing without at the same time seeing another, different objects hang together in recollection, and when we call for one the other quickly follows. To think of the place in which we have last seen a thing is often the best way of recalling it to the mind. Hence memory is dependent on association. The act of recollection may be compared to the sight of an object at a great distance which we have previously seen near and seek to bring near to us in thought. Memory is to sense as dreaming is to waking; and like dreaming has a wayward and uncertain power of recalling impressions from the past. Thus begins the passage from the outward to the inward sense. But as yet there is no conception of a universal--the mind only remembers the individual object or objects, and is always attaching to them some colour or association of sense. The power of recollection seems to depend on the intensity or largeness of the perception, or on the strength of some emotion with which it is inseparably connected. This is the natural memory which is allied to sense, such as children appear to have and barbarians and animals. It is necessarily limited in range, and its limitation is its strength. In later life, when the mind has become crowded with names, acts, feelings, images innumerable, we acquire by education another memory of system and arrangement which is both stronger and weaker than the first--weaker in the recollection of sensible impressions as they are represented to us by eye or ear--stronger by the natural connexion of ideas with objects or with one another. And many of the notions which form a part of the train of our thoughts are hardly realized by us at the time, but, like numbers or algebraical symbols, are used as signs only, thus lightening the labour of recollection.