Our sense-impressions are marked off from all other feelings by having an objective character, that is to say, an immediate relation to the external world, so that in attending to one of them our minds pass away from themselves in what Professor Bain calls the attitude of objective regard.

Introspection is confined to feelings which want this intimate connection with the external region, and includes sensation only so far as it is viewed apart from external objects and on its mental side as a feeling, a process which is next to impossible where the sensation has little emotional colour, as in the case of an ordinary sensation of sight or of articulate sound.
This being so, errors of introspection, supposing such to be found, will in the main be sufficiently distinguished from those of perception.

Even an hallucination of sense, whether setting out from a subjective sensation or not, always contains the semblance of a sense-impression, and so would not be correctly classed with errors of introspection.
Just as introspection must be marked off from perception, so must it be distinguished from memory.

It may be contended that, strictly speaking, all introspection is retrospection, since even in attending to a present feeling the mind is reflectively representing to itself the immediately preceding momentary experience of that feeling.

Yet the adoption of this view does not hinder us from drawing a broad distinction between acts of introspection and acts of memory.