These sensations occurring together so frequently, blend into one, and so we infer, according to the general instinctive tendency already noticed, that there is one specific quality answering to the feeling.

And since the feeling is nearly always produced by surfaces moistened by cold liquid, we refer it to this circumstance, and speak of it as a feeling of wetness.

Hence, when the particular conjunction of sensations arises apart from this external circumstance, we erroneously infer its presence.[22] The most interesting case of illusion connected with the fusion of simultaneous sensations, is that of single vision, or the deeply organized habit of combining the sensations of what are called the corresponding points of the two retinas.

This coalescence of two sensations is so far erroneous since it makes us overlook the existence of two distinct external agencies acting on different parts of the sensitive surface of the body.

And this is the more striking in the case of looking at solid objects, since here it is demonstrable that the forces acting on the two retinas are not perfectly similar.
Nevertheless, such a coalescence plainly answers to the fact that these external agencies usually arise in one and the same object, and this unity of the object is, of course, the all-important thing to be sure of.
This habit may, however, beget palpable illusion in another way.