If it be insisted that the 'sensation in my thumb' and that in my first finger are at any rate 'signs' of the distance, how could these sensations come to have in themselves any means of signifying the relationship between points in space, unless they were already situated on a path running from one to the other, and unless this path in its turn were not only traversed by my fingers as they open, but also 'aimed at' by my thought pursuing its intelligible purpose? '