Another reason, however, must be sought for the habit of assimilating internal feelings to external sensations.  If language has been evolved as an incident of social life, at once one of its effects and its causes, it would seem to follow that it must have first shaped Itself to the needs of expressing these common objective experiences which we receive by way of our senses.  Our habitual modes of thought, limited as they are by language, retain traces of this origin.  We cannot conceive any mental process except by some vague analogy to a physical process.  In other words, we can even now only think with perfect clearness when we are concerned with some object of common cognition.  Thus, the sphere of external sensation and of physical agencies furnishes us with the one type of thinkable thing or object of thought, and we habitually view subjective mental states as analogues of these.

Still, it may be said that these slight nascent errors are hardly worth naming, and the question would still appear to recur whether there are other fully developed errors deserving to rank along with illusions of sense.  Do we, it may be asked, ever actually mistake the quality, degree, or structure of our internal feelings in the manner hinted above, and if so, what is the range of such error?  In order to appreciate the risks of such error, let us compare the process of self-observation with that of external perception with respect to the difficulties in the way of accurate presentative knowledge.

Misreading of Internal Feelings.

First of all, it is noteworthy that a state of consciousness at any one moment is an exceedingly complex thing.  It is made up of a mass of feelings and active impulses which often combine and blend in a most inextricable way.  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.