Thus, the feeling of something creeping over the skin is an hallucination in the sense that it implies the idea of an object external to the skin.
Similarly, the projection of an ocular impression due to retinal disturbance into the external field of vision, may rightly be named an hallucination.

But the case is not always so clear as this.

Thus, for example, when a gustatory sensation is the result of an altered condition of the saliva, it may be said that the error is as much an illusion as an hallucination.[31] In a wide sense, again, all errors connected with those subjective sensations which arise from a stimulation of the peripheral regions of the nerve may be called illusions rather than hallucinations.

Or, if they must be called hallucinations, they may be distinguished as "peripheral" from those "central" hallucinations which arise through an internal automatic excitation of the sensory centre.

It is plain from this that the region of subjective sensation is an ambiguous region, where illusion and hallucination mix and become confused.