Any one who has paid attention to the curious subject of what are called "subjective sensations" will be familiar with examples of the extreme difficulty which sometimes attends the discrimination of ideas of sensation from impressions of sensation, when the ideas are very vivid, or the impressions are faint.

Who has not "fancied" he heard a noise; or has not explained inattention to a real sound by saying, "I thought it was nothing but my fancy"?
Even healthy persons are much more liable to both visual and auditory spectra--that is, ideas of vision and sound so vivid that they are taken for new impressions--than is commonly supposed; and, in some diseased states, ideas of sensible objects may assume all the vividness of reality.
If ideas are nothing but copies of impressions, arranged, either in the same order as that of the impressions from which they are derived, or in a different order, it follows that the ultimate analysis of the contents of the mind turns upon that of the impressions.

According to Hume, these are of two kinds: either they are impressions of sensation, or they are impressions of reflection.

The former are those afforded by the five senses, together with pleasure and pain.

The latter are the passions or the emotions (which Hume employs as equivalent terms).