On his own principles, Hume should therefore have placed this "perception" among the ideas of reflection.

However, as we have seen, he expressly excludes everything but the emotions and the passions from this group.
It is necessary therefore to amend Hume's primary "geography of the mind" by the excision of one territory and the addition of another; and the elementary states of consciousness will stand thus:-- A.IMPRESSIONS.
A.Sensations of _a._ Smell.
_b._ Taste.
_c._ Hearing.
_d._ Sight.
_e._ Touch.
_f._ Resistance (the muscular sense).
B.Pleasure and Pain.
C.Relations.
_a._ Co-existence.
_b._ Succession.
_c._ Similarity and dissimilarity.
B.IDEAS.
Copies, or reproductions in memory, of the foregoing.
And now the question arises, whether any, and if so what, portion of these contents of the mind are to be termed "knowledge." According to Locke, "Knowledge is the perception of the agreement or disagreement of two ideas;" and Hume, though he does not say so in so many words, tacitly accepts the definition.

It follows, that neither simple sensation, nor simple emotion, constitutes knowledge; but that, when impressions of relation are added to these impressions, or their ideas, knowledge arises; and that all knowledge is the knowledge of likenesses and unlikenesses, co-existences and successions.
It really matters very little in what sense terms are used, so long as the same meaning is always rigidly attached to them; and, therefore, it is hardly worth while to quarrel with this generally accepted, though very arbitrary, limitation of the signification of "knowledge." But, on the face of the matter, it is not obvious why the impression we call a relation should have a better claim to the title of knowledge, than that which we call a sensation or an emotion; and the restriction has this unfortunate result, that it excludes all the most intense states of consciousness from any claim to the title of "knowledge." For example, on this view, pain, so violent and absorbing as to exclude all other forms of consciousness, is not knowledge; but becomes a part of knowledge the moment we think of it in relation to another pain, or to some other mental phenomenon.

Surely this is somewhat inconvenient, for there is only a verbal difference between having a sensation and knowing one has it: they are simply two phrases for the same mental state.
But the "pure metaphysicians" make great capital out of the ambiguity.
For, starting with the assumption that all knowledge is the perception of relations, and finding themselves, like mere common-sense folks, very much disposed to call sensation knowledge, they at once gratify that disposition and save their consistency, by declaring that even the simplest act of sensation contains two terms and a relation--the sensitive subject, the sensigenous object, and that masterful entity, the Ego.

From which great triad, as from a gnostic Trinity, emanates an endless procession of other logical shadows and all the _Fata Morgana_ of philosophical dreamland.
FOOTNOTES: [18] "Consciousnesses" would be a better name, but it is awkward.