Putting ourselves back to the Platonic period, we must recognise that the discussion of the theory [Greek: Ai)/sthêsis = E)pistê/mê], as it is conducted by Plato, exhibits a remarkable advance in psychological analysis. In analysing the mental phenomena, Plato displayed much more subtlety and acuteness than his predecessors--as far at least as we have the means of appreciating the latter. It is convenient to distinguish intellect from sensation (or sensible perception) and emotion, though both of them are essential and co-ordinate parts of our mental system, and are so recognised by Plato. It is also true that the discrimination of our sensations from each other, comparisons of likeness or unlikeness between them, observation of co-existence or sequence, and apprehension of other relations between them, &c., are more properly classified as belonging to intellect than to sense. But the language of psychology is, and always has been, so indeterminate, that it is difficult to say how much any writer means to include under the terms Sense[99]--Sensation--Sensible Perception--[Greek: Ai)/sthêsis]. The propositions in which our knowledge is embodied, affirm--not sensations detached and isolated, but--various relations of antecedence and consequence, likeness, difference, &c., between two or more sensations or facts of sense. We rise thus to a state of mind more complicated than simple sensation: including (along with sensation), association, memory, discrimination, comparison of sensations, abstraction, and generalisation. This is what Plato calls opinion[100] or belief; a mental process, which, though presupposing sensations and based upon them, he affirms to be carried on by the mind through itself, not through any special bodily organ. In this respect it agrees with what he calls knowledge or cognition. Opinion or belief is the lowest form, possessed in different grades by all men, of this exclusively mental process: knowledge or cognition is the highest form of the same, attained only by a select few. Both opinion, and cognition, consist in comparisons and computations made by the mind about the facts of sense. But cognition (in Plato's view) has special marks:-- 1. That it is infallible, while opinion is fallible. You have it[101] or you have it not--but there is no mistake possible. 2. That it apprehends what Plato calls the real essence of things, and real truth, which, on the contrary, Opinion does not apprehend. 3. That the person who possesses it can maintain his own consistency under cross-examination, and can test the consistency of others by cross-examining them ([Greek: lo/gon dou=nai kai\ de/xasthai]).