From simple as well as complex sensations, we form judgments which I will call simple ideas. In a sensation the judgment is wholly passive, only affirming that we feel what we feel. In a preception or idea, the judgment is active; it brings together, compares, and determines relations not determined by the senses. This is the only point of difference, but it is important. Nature never deceives us; it is always we who deceive ourselves.
I see a child eight years old helped to some frozen custard. Without knowing what it is, he puts a spoonful in his mouth, and feeling the cold sensation, exclaims, "Ah, that burns!" He feels a keen sensation; he knows of none more so than heat, and thinks that is what he now feels. He is of course mistaken; the chill is painful, but does not burn him; and the two sensations are not alike, since, after encountering both, we never mistake one for the other. It is not, therefore, the sensation which misleads him, but the judgment based on it.
It is the same when any one sees for the first time a mirror or optical apparatus; or enters a deep cellar in mid-winter or midsummer; or plunges his hand, either very warm or very cold, into tepid water; or rolls a little ball between two of his fingers held crosswise. If he is satisfied with describing what he perceives or feels, keeping his judgment in abeyance, he cannot be mistaken. But when he decides upon appearances, his judgment is active; it compares, and infers relations it does not perceive; and it may then be mistaken. He will need experience to prevent or correct such mistakes. Show your pupil clouds passing over the moon at night, and he will think that the moon is moving in an opposite direction, and that the clouds are at rest. He will the more readily infer that this is the case, because he usually sees small objects, not large ones, in motion, and because the clouds seem to him larger than the moon, of whose distance he has no idea. When from a moving boat he sees the shore at a little distance, he makes the contrary mistake of thinking that the earth moves. For, unconscious of his own motion, the boat, the water, and the entire horizon seem to him one immovable whole of which the moving shore is only one part.