But is not this the very thing we were in search of? have we not at last got at the secret of visible depth? No; we are rather _at the crucial point of our discussion_. For this binocular picture, the image seen in the stereoscope, cannot be, of its own nature and in its own right, deep or solid, unless there is a depth-sensation; and that conclusion goes against everything that we know both of sensation and of the stimuli that arouse sensation. To avoid it, some psychologists call in the kinæsthetic sensations from the muscles of the eye. _Depth or distance, they say, is psychologically a blend or fusion of visual and kinæsthetic sensations._ Our binocular view of the book, its appearance to the two eyes, is in itself flat; but we run the eyes over it, and the muscular sensations thus blend with the visual. Nay more, even if we hold our eyes fixed, there is still a _tendency_ to move them; and this tendency, now ingrained in our nervous system, is enough to realise the perception. Indeed, if experiment fails in every case to show the sensations of convergence and accommodation, that is just because the fusion is so long-established and so ingrained; we perceive distance, the fusion itself; we can hardly expect to recover the kinæsthetic sensations that originally entered into it; the wonder rather is that they should ever appear, that experiment should be able to reveal them at all. No one can say positively that this hypothesis is wrong; but it is difficult to believe that the blend of visual and kinæsthetic sensations should yield a result so different from either,—namely, the perception of space. It seems safer to say that _the binocular picture_, the appearance of the book to the two eyes or the combined image of the stereoscope, _carries the immediate meaning of depth or voluminousness_. The picture is not itself deep or solid; but we cannot help perceiving it _as_ deep and solid; and this pressure is laid upon us by what we have called racial heritage, an inherited disposition of the nervous system: _the brain meets the impression halfway_. The binocular picture thus becomes the core or nucleus of the perception; and the meaning of depth is carried by a nervous disposition that has no correlate in sensation or image. The kinæsthetic sensations may then very well serve, as a secondary context, to give precision and accuracy to the perception, to develop the perception of crude voluminousness into the perception of definite distances. As to the nervous disposition, we can only say that it has been set up by the same biological causes that have made the organism a motor organism, one that moves freely in space; beyond that general statement we cannot go.