own extremities; for, as every primer of psychology will teach you, mere sight and its muscular adjustments account only for the dimensions of height (up and down) and of breadth (right and left) while the third or cubic dimension of _depth_ is a highly complex result of locomotion in which I include prehension. And inasmuch as we are dealing with _aspects_ and not with _things,_ we have as yet nothing to do with this _cubic_ or _third dimension,_ but are confining ourselves to the two dimensions of extension in height and breadth, which are sufficient for the existence, the identity, or more correctly the _quiddity,_ of visible shapes. Such a shape is therefore, primarily, a series of longer or shorter _extensions,_ given by a separate glance towards, or away from, our own centre or extremities, and at some definite angle to our own axis and to the ground on which we stand. But these acts of extension and orientation cease to be thought of as measured and orientated, and indeed as accomplished, by ourselves, and are translated into objective terms whenever our attention is turned outwards: thus we say that each line is of a given length and direction, so or so much off the horizontal or vertical. So far we have established relations only to ourselves. We now compare the acts of extension one against the other, and we also measure the adjustment requisite to pass from one to another, continuing to refer them all to our own axis and centre; in everyday speech, we perceive that the various lines are _similar_ and _dissimilar_ in length, direction and orientation. We _compare;_ and comparing we _combine_ them in the unity of our intention: thought of together they are thought of as belonging together. Meanwhile the process of such comparison of the relation of each line with us to the analogous relation to us of its fellows, produces yet further acts of measurement and comparison. For in going from one of our lines to another we become aware of the presence of--how shall I express it?--well of a _nothing_ between them, what we call _blank space,_ because we experience a _blank_ of the particular sensations, say red and black, with which we are engaged in those lines. Between the red and black sensations of the lines we are looking at, there will be a possibility of other colour sensations, say the white of the paper, and these white sensations we shall duly receive, for, except by shutting our eyes, we could not avoid receiving them. But though received these white sensations will not be attended to, because they are not what we are busied with. We shall be _passive_ towards the white sensations while we are _active_ towards the black and red ones; we shall not measure the white; not sweep our glance along it as we do along the red and the black. And as _ceteris paribus_ our tense awareness of active states always throws into insignificance a passive state sandwiched between them; so, bent as we are upon our red and black extensions,