Such qualities as _stickiness_, _roughness_, etc., are supposed to be felt through the coöperation of muscular sensations with those of the skin. The geometrical qualities of things, on the other hand, their _shapes_, _bignesses_, _distances_, etc. (so far as we discriminate and identify them), are by most psychologists supposed to be impossible without the evocation of memories from the past; and the cognition of these attributes is thus considered to exceed the power of sensation pure and simple. ='Knowledge of Acquaintance' and 'Knowledge about.'=--Sensation, thus considered, differs from perception only in the extreme simplicity of its object or content. Its object, being a simple quality, is sensibly _homogeneous_; and its function is that of mere _acquaintance_ with this homogeneous seeming fact. Perception's function, on the other hand, is that of knowing something _about_ the fact. But we must know _what_ fact we mean, all the while, and the various _whats_ are what sensations give. Our earliest thoughts are almost exclusively sensational. They give us a set of _whats_, or _thats_, or _its_; of subjects of discourse in other words, with their relations not yet brought out. The first time we see _light_, in Condillac's phrase we _are_ it rather than see it. But all our later optical knowledge is about what this experience gives. And though we were struck blind from that first moment, our scholarship in the subject would lack no essential feature so long as our memory remained. In training-institutions for the blind they teach the pupils as much _about_ light as in ordinary schools. Reflection, refraction, the spectrum, the ether-theory, etc., are all studied. But the best taught born-blind pupil of such an establishment yet lacks a knowledge which the least instructed seeing baby has. They can never show him _what_ light is in its 'first intention'; and the loss of that sensible knowledge no book-learning can replace. All this is so obvious that we usually find sensation 'postulated' as an element of experience, even by those philosophers who are least inclined to make much of its importance, or to pay respect to the knowledge which it brings.