In short, we get into the way of attending only to what is essential, constant, and characteristic in objects, and disregarding what is variable and accidental.[46] Thus, we attend, in the first place, to the form of objects, the most constant and characteristic element of all, being comparatively inattentive to colour, which varies with distance, atmospheric changes, and mode of illumination.

So we attend to the relative magnitude of objects rather than to the absolute, and to the relative intensities of light and shade rather than to the absolute; for in so doing we are noting what is constant for all distances and modes of illumination, and overlooking what is variable.

And the success of pictorial art depends on the observance of this law of perception.
These remarks at once point out the limits of these illusions.

In normal circumstances, an act of imagination, however vivid, cannot create the semblance of a sensation which is altogether absent; it can only slightly modify the actual impression by interfering with that process of comparison and classification which enters into all definite determination of sensational quality.
Another great fact that has come to light in the investigation of these illusions is that oft-recurring and familiar types of experience leave permanent dispositions in the mind.

As I said when describing the process of perception, what has been frequently perceived is perceived more and more readily.