Supposing the universe to consist of atoms separated by very fine intervals, then it is demonstrable that it would appear to our sensibility as a continuum, just as it does now.[19] Two or more simultaneous sensations are indistinguishable from one another, not only when they have nearly the same local origin, but under other circumstances.

The blending of partial sensations of tone in a _klang_-sensation, and the coalescence in certain cases of the impressions received by way of the two retinas, are examples of this.

It is not quite certain what determines this fusion of two simultaneous feelings.

It may be said generally that it is favoured by similarity between the sensations;[20] by a comparative feebleness of one of the feelings; by the fact of habitual concomitance, the two sensations occurring rarely, if ever, in isolation; and by the presence of a mental disposition to view them as answering to one external object.

These considerations help us to explain the coalescence of the retinal impressions and its limits, the fusion of partial tones, and so on.[21] It is plain that this fusion of sensations, whatever its exact conditions may be, gives rise to error or wrong interpretation of the sense-impression.