The consideration of objects introduces the concepts of 'matter'—or more vaguely, 'material'—of 'transmission' and of 'causation.' These concepts express certain relations of objects to events, but the relations are too complex to be fully expressed in such simple terms. 22.2 The essence of the perception of an object is recognition. There is the primary recognition which is the awareness of permanence within the specious present; there is the indefinite recognition (which we may term 'recollection') which is the awareness of other perceptions of the object as related to other events separate from the specious present, but without any precise designation of the events; and there is the definite recognition (which we may term 'memory') which is an awareness of perception of the object as related to certain other definite events separate from the specious present. 22.3 The awareness of external nature is an awareness of a duration, which is the being of nature throughout the specious present, and of a complex of events, each being part of the present duration. These events fall into two sets. In one set is the percipient event and in the other are the external events whose peculiar property, which has led to their discernment, is that they are the situations of sense-objects. 22.4 The percipient event is discerned as the locus of a recognisable permanence which is the 'percipient object.' This object is the unity of the awareness whose recognition leads to the classification of a train of percipient events as the natural life associated with one consciousness. The discussion of the percipient object leads us beyond the scope of this enquiry. Owing to the temporal duration of the immediate present the self-knowledge of the percipient object is a knowledge of the unity of the consciousness within other parts of the immediate present. Thus, though it is a knowledge of what is immediately present, it is not a knowledge knowing itself. 23. Sense-Objects. 23.1 The sense-object is the simplest permanence which we trace as self-identical in external events. It is some definite sense-datum, such as the colour red of a definite shade. We see redness here and the same redness there, redness then and the same redness now. In other words, we perceive redness in the same relation to various definite events, and it is the same redness which we perceive. Tastes, colours, sounds, and every variety of sensation are objects of this sort. 23.2 There is no apprehension of external events apart from recognitions of sense-objects as related to them, and there is no recognition of sense-objects except as in relation to external events.