less faint of the whole line on which they lie. In other words, no ray of light can fall on any retinal spot without the local sign of that spot revealing to us, by recalling the line of its most habitual associates, its direction and distance from the centre of the field. The fovea acts thus as the origin of a system of polar co-ordinates, in relation to which each and every retinal point has through an incessantly-repeated process of association its distance and direction determined. Were _P_ alone illumined and all the rest of the field dark we should still, even with motionless eyes, know whether _P_ lay high or low, right or left, through the _ideal streak_, different from all other streaks, which _P_ alone has the power of awakening.[167] And with this we can close the first great division of our subject. We have shown that, within the range of every sense, experience takes _ab initio_ the spatial form. We have also shown that in the cases of the retina and skin every sensible total may be subdivided by discriminative attention into sensible parts, which are also spaces, and into relations between the parts, these being sensible spaces too. Furthermore, we have seen (in note 167) that different parts, once discriminated, necessarily fall into a determinate order, both by reason of definite gradations in their quality, and by reason of the fixed order of time-succession in which movements arouse them. But in all this nothing has been said of the comparative _measurement_ of one sensible space-total against another, or of the way in which, by summing our divers simple sensible space-experiences together, we end by constructing what we regard as the unitary, continuous, and infinite objective Space of the real world. To this more difficult inquiry we next pass. THE CONSTRUCTION OF 'REAL' SPACE. The problem breaks into two subordinate problems. (1) _How is the subdivision and measurement of the several sensorial spaces completely effected?_ and (2) _How do their mutual addition and fusion and reduction to the same scale, in a word, how does their synthesis, occur?_ I think that, as in the investigation just finished, we found ourselves able to get along without invoking any data but those that pure sensibility on the one hand, and the ordinary intellectual powers of discrimination and recollection on the other, were able to yield; so here we shall emerge from our more complicated quest with the conviction that all the facts can be accounted for on the supposition that no other mental forces have been at work save those we find everywhere else in psychology: sensibility, namely, for the data; and discrimination, association, memory, and choice for the rearrangements and combinations which they undergo. 1. _The Subdivision of the Original Sense-spaces._