PERCEPTION OF PHYSICAL THINGS The Problem.—The second of the general problems of cog nition is to account for the growth of our apprehension of a world of physical things and of the self in relation to which this world is described as "external." In this development three distinct levels would seem to be involved. There is the normal adult human perception of daily life. Below it there is a much more primitive level of perceptual apprehension, and above, the level of conceptual thought and ideal construction. Through a window I perceive a not too distant mountain. In spite of appearances I take it to be of enormous size, and to be covered with white snow. This is the normal interpretation of perception. But it appears extremely small and, on account of the setting sun, of a distinctly reddish hue. These are the perceptual data. But neither of these accounts would seem to correspond with "scientific fact." The apparently solid and motionless mountain is explained as a whirl ing mass of electrons, and endowed with properties which I neither do nor can perceive nor directly infer from what I per ceive. The immediate problem, however, is to disentangle the earlier levels of apprehension.
Primary Data.—Whatever else may be involved sensible ex perience must be accepted as primary. But mere sense-presenta tions strictly and abstractly conceived alone are not enough. The processes we have considered, operative only with such ma terial, would not suffice to explain our apprehension of a world of physical things. It must be taken as a datum for Psychology— as a fact of experience—that we are cognisant of and active about objects which are not themselves capable of being sensibly experienced. Thought or judgment transcends not only present sense-experience but sense-experience in general.
So far as cognition is concerned thought may be psychologi cally considered as a tissue of beliefs and suppositions. Belief as a mental function is an irreducible component of experience, but particular beliefs are primary in another sense. The majority of beliefs are inferential or arise from mere suggestion. Beliefs not so derived may be described as "original," and original beliefs must be involved in the explanation of our apprehension of the material world.
Corresponding to every sense-presentation, involving an act of attention, there arises an original belief that something is there and is characterised in a more or less definite way. There are orig inal judgments involved in recognition, and memory judgments asserting past experiences, as such, may similarly be "original." Discrimination between original and derivative beliefs can only be effected in the course of a detailed investigation of the development of cognition. One very general principle, however, would seem to be involved in all the fundamental modes in which the mind refers to its objects. Such reference is conditioned by sense-experience. Every variation in the thought depends upon some variation in sense and every variation in sense-experience makes possible some variation in thought. Hence therefore, paral lel to, and based upon, the development of sensory experience, thought advances in complexity, determination and range. The simplest presentation is endowed with primary meaning, second ary meaning being acquired through retention. Retention, asso ciation and reproduction of thought, depend upon analogous proc esses in the sensory field. The parallelism, however, is subject to complications in detail. Sometimes likeness among sense-pres entations means only a corresponding likeness among the objects to which thought refers; sometimes likeness of presentations pro vides the basis for a judgment of identity. Change in the sensory field points to changes in its conditions, but not necessarily in the object singled out as the thing that is being perceived. Sensible movement in immediate experience conditions the primary judg ment "something moves." According to the context the move ment may be referred to various things, to the things which appear to move, to the otherwise unperceived medium or to the bodily self. In fact the explanation of the development of cognition con sists in one aspect (the abstractly cognitive) in determining the nature of the sensory experience which conditions our developing apprehension.