This is the most commonly employed meaning of the term spatial. Yet it is evidently in its origin rather temporal than spatial. In ordinary movement we encounter by touch various obstacles, but only a very few of these impress us at any one moment of time. On the contrary, they succeed one after the other. To the blind, therefore, as Platner long ago remarked: Time serves instead of Space. In Vision, on the other hand, a large number, which it would take a very long time to encounter in touch, are presented _simultaneously_. In this there is an immense practical advantage, the result being that we come habitually to direct our every action by reference to the data of Sight. Now it is because these data--so simultaneously presented--are employed by us as the guides of action that their presentation acquires the character which we denominate Extensity. The simultaneous occurrence of a large number of Sounds does not seem to us to present such a character. But let us suppose that all the objects which constitute obstacles to our Activity emitted Sounds by which they were recognised; it is not doubtful that these would then come to be employed by us as the guides of our Activity and would acquire in our minds the character of Extensity. They would arrange themselves in a cotemporaneous, extensive, or spatial relation to one another just as the objects of Vision do at present. It is only, therefore, when we come to employ the simultaneous presentation of Vision as the instrument of our Activity and the guide of Action that it acquires the character commonly called extensive. _Successive_ visual sensations convey no extensive suggestion. It is important to realise the nature of this peculiar feature in the data of Vision. The sounds which we hear, the odours which we smell, are the immediate result of certain undulations affecting the appropriate organ of sensation. We refer these to the object in which the undulations originate. In like manner a light which we see is referred to its objective luminous source. But light also and in addition is reflected from, and thus reveals the presence of the whole body of our resistant environment. Hence is derived the coloured presentation of Vision to which the character of extensity attaches. Nothing similar takes place in the case of the other distantial sensations. If sonorous undulations excited vibration in every resistant object of the environment they would undoubtedly come to arrange themselves in an order resembling the extensity suggested by Vision, though the slower rate of transmission of sound would detract from the practical simultaneity in the effect which, as we have seen, largely accounts for the perception of visual extensity. The universal diffusion of sunlight is also a determining factor. * * * * *