=Perception an Acquired Power.=--On the other hand, however, investigation will show that this power to recognize explicitly the existence of an external object through the presentation of a sensation, was not at first possessed by the mind. The ability thus to perceive objects represents, therefore, an acquirement on the part of the individual. If a person, although receiving merely sensations of colour and light, is able to say, "Yonder is an orange," he is evidently interpreting, or giving meaning to, the present sensations largely through past experience; for the images of colour and light are accepted by the mind as an indication of the presence of an external thing from which could be derived other images of taste, smell, etc., all of which go to make up the idea "orange." An ordinary act of perception, therefore, must involve not merely sensation, but also an interpretation of sensation through past experience. It is, in fact, because the recognition of an external object involves this conscious interpretation of the sensuous impressions, that people often suffer delusion. When the traveller passing by a lone graveyard interprets the tall and slender shrub laden with white blossoms as a swaying ghost, the misconception does not arise from any fault of mere vision, but from the type of former knowledge which the other surroundings of the moment call up, these evidently giving the mind a certain bias in its interpretation of the sensuous, or colour, impressions. =Perception in Adult Life.=--In our study of general method, sense perception was referred to as the most common mode of acquiring particular knowledge. A description of the development of this power to perceive objects through the senses should, therefore, prove of pedagogical value. But to understand how an individual acquires the ability to perceive objects, it is well to notice first what takes place in an ordinary adult act of perception, as for instance, when a man receives and interprets a colour stimulus and says that he perceives an orange. If we analyse the person's idea of an orange we find that it is made up of a number of different quality images--colour, taste, smell, touch, etc., organized into a single experience, or idea, and accepted as a mental representation of an object existing in space. When, therefore, the person referred to above says that he perceives an orange, what really happens is that he accepts the immediate colour and light sensation as a sign of the whole group of qualities which make up his notion of the external object, orange, the other qualities essential to the notion coming back from past experience to unite with the presented qualities. Owing to this fact, any ordinary act of perception is said to contain both presentative and representative elements. In the above example, for instance, the colour would be spoken of as a presentative element, because it is immediately presented to the mind in sensuous terms, or through the senses.