External sensations come in groups, too, but as a rule they do not fuse in apparently simple wholes as our internal feelings often do.

The very possibility of perception depends on a clear discrimination of sense-elements, for example, the several sensations of colour obtained by the stimulation of different parts of the retina.[103] But no such clearly defined mosaic of feelings presents itself in the internal region: one element overlaps and partly loses itself in another, and subjective analysis is often an exceedingly difficult matter.

Our consciousness is thus a closely woven texture in which the mental eye often fails to trace the several threads or strands.

Moreover, there is the fact that many of these ingredients are exceedingly shadowy, belonging to that obscure region of sub-consciousness which it is so hard to penetrate with the light of discriminative attention.

This remark applies with particular force to that mass of organic feelings which constitutes what is known as coenaesthesis; or vital sense.
While, to speak figuratively, the minute anatomy of consciousness is thus difficult with respect to longitudinal sections of the mental column, it is no less difficult with respect to transverse sections.
Under ordinary circumstances, external impressions persist so that they can be transfixed by a deliberate act of attention, and objects rarely flit over the external scene so rapidly as to allow us no time for a careful recognition of the impression.