On the subject of perception, it is relevant to observe that the ideas we receive by sensation are often altered in grown people by the judgment of the mind, without our taking any notice of it. Thus when we set before our eyes a round body of uniform color, of gold, alabaster, or jet for example, it is certain that the idea that is imprinted in our mind at the sight of this globe represents a flat circle, variously shadowed, with different degrees of light coming to our eyes. But having by use been accustomed to distinguish the sort of images that convex bodies are wont to produce in us and the alterations that occur in the reflection of light, by the sensible difference in the bodies, we right away, for what appears to us, substitute the very cause of the image we see by virtue of a judgment which custom has made habitual with us; so that joining to what we see a judgment that we confuse with it, we make our own idea of a convex figure and a uniform color, though the eyes actually represent to us only a flat surface variously shaded and colored, as it would appear in painting. […]