Perception may be defined as the cognizance which the mind takes of impressions presented to it through the organs of sense, and possibly also by other means. One class of perceptions is universally recognized and is in a measure understood, namely, perceptions arising from impressions made by recognized external objects or forces upon the organs of sense, sight, hearing, smell, taste, and also the general sense of touch. These perceptions in particular are designated as _real_ or _true_, because they correspond to recognized external realities. But impressions are also made upon the organs of special sense by influences which are not recognized as having any objective reality, but which nevertheless affect the senses in a manner often identical with that in which they are affected by recognized external objects, and they cause the same perceptions to arise in the mind. Hence another broad class of perceptions includes those which are taken cognizance of by the mind from impressions made upon the organs of sense in other ways and by other means than by external objects, and often where there is no evidence that any external object exists corresponding to the impression so made. Perceptions arising in these various ways are called _hallucinations_. On close examination, however, it is found that the sharp line of separation between what has and what has not an objective reality is not easily drawn, any more than in biology the sharp line between animal and vegetable life can be easily drawn, or at the lower end of the scale between the living and the not living. So the origin of those perceptions which are classed as hallucinations has always been a subject of controversy, even among philosophers of the greatest merit and eminence. Without following out the discussions which have arisen on this point--discussions which are often confusing and generally inconclusive, a fairly distinct view of the subject may be obtained by considering the origin of these perceptions under three heads--namely:-- (1) Perceptions which are reckoned as hallucinations may be originated _centrally_; that is, they may arise wholly within the mind itself without any direct external stimulus. For instance the characters drawn by the novelist may become so real to him, and even to some of his readers, that they become _externalized_--actual objects of visual perception and are seen to act and even heard to speak. The instance is repeatedly quoted of the painter who, after carefully studying a sitter's appearance, could voluntarily project it visibly into space and paint the portrait, not from the original, but from the phantasm so produced; and of another who could externalize and project other mental pictures in the same manner, pictures which so interested him and were so subject to the ordinary laws of vision that he would request any one who took a position in front of them, to move away so as not to obstruct his view.