§ 1. Introductory. — We now pass from perceptual to ideational process, — from those trains of mental activity which are prompted and guided by external impressions and directly worked out in bodily movement to those which proceed independently of external stimulation and are worked out "in the head." Up to this point we have taken into account ideas or images only in so far as they enter into the composition of processes which are in their essential character perceptual: we have now to consider processes which reach their end through mental images succeeding each other in a series independently of actual perception. Before expounding the distinctive nature and function of this higher mode of mental activity, it is necessary to examine with some care the characteristic features of a mental image. In what respects does an object as merely imaged differ from the same object as actually perceived?

It should be clearly understood that those visual experiences which are called "afterimages "both positive and negative are in reality "after-sensations." They are due to the continued excitement of the organ of sense after the external stimulus has ceased to operate, and cannot therefore be regarded as ideas. They are easily distinguishable from what has been called the primary memoryimage. This is the peculiarly vivid and definite ideal representation of an object which we can maintain or recall by a suitable effort of attention immediately after perceiving it. The persistence of the afterimage does not depend on an effort of attention, but on the abiding effect of the external stimulus. It passes, for the most part, very rapidly from a positive to a negative phase and undergoes other modifications which do not affect the primary memoryimage. There is also another conspicuous and important distinction: whatever may have been the spatial arrangement of the perceived objects, the corresponding afterimages are spread out in a flat expanse; but the solidity and perspective of objects as actually seen reappear in the primary memoryimage and in ideal images generally.