_Sensation, then, so long as we take the analytic point of view, differs from Perception only in the extreme simplicity of its object or content._[1] Its function is that of mere _acquaintance_ with a fact. Perception's function, on the other hand, is knowledge _about_[2] a fact; and this knowledge admits of numberless degrees of complication. But in both sensation and perception we perceive the fact as an _immediately present outward reality_, and this makes them differ from 'thought' and 'conception,' whose objects do not appear present in this immediate physical way. _From the physiological_ _point of view both sensations and perceptions differ from 'thoughts'_ (in the narrower sense of the word) _in the fact that nerve-currents coming in from the periphery are involved in their production. In perception these nerve-currents arouse voluminous associative or reproductive processes in the cortex; but when sensation occurs alone, or with a minimum of perception, the accompanying reproductive processes are at a minimum too._ I shall in this chapter discuss some general questions more especially relative to Sensation. In a later chapter perception will take its turn. I shall entirely pass by the classification and natural history of our special 'sensations,' such matters finding their proper place, and being sufficiently well treated, in all the physiological books.[3] THE COGNITIVE FUNCTION OF SENSATION. _A pure sensation is an abstraction;_ and when we adults talk of our 'sensations' we mean one of two things: either certain _objects_, namely simple _qualities_ or _attributes_ like _hard, hot, pain;_ or else those of our thoughts in which acquaintance with these objects is least combined with knowledge about the relations of them to other things. As we can only think or talk about the relations of objects with which we have _acquaintance_ already, we are forced to postulate a function in our thought whereby we first become aware of the _bare immediate natures_ by which our several objects are distinguished. This function is sensation. And just as logicians always point out the distinction between substantive terms of discourse and relations found to obtain between them, so psychologists, as a rule, are ready to admit this function, of the vision of the terms or matters meant, as something distinct from the knowledge about them and of their relations _inter se_. Thought with the former function is sensational, with the latter, intellectual. Our earliest thoughts are almost exclusively sensational. They merely give us a set of _thats_, or _its_, of subjects of discourse, with their relations not brought out. The first time we see _light_, in Condillac's phrase we _are_ it rather 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.