Thus, the more frequently a similar process of perception has been performed in the past, the more ready will the mind be to fall into the particular way of interpreting the impression.

As G.H.Lewes well remarks, "The artist sees details where to other eyes there is a vague or confused mass; the naturalist sees an animal where the ordinary eye only sees a form." This is but one illustration of the seemingly universal mental law, that what is repeatedly done will be done more and more easily.
The process of preperception may be shortened, not only by means of a _permanent_ disposition to frame the required interpretative scheme, the residuum of past like processes, but also by means of any _temporary_ disposition pointing in the same direction.

If, for example, the mind of a naturalist has just been occupied about a certain class of bird, that is to say, if he has been dwelling on the _mental image_ of this bird, he will recognize one at a distance more quickly than he would otherwise have done.

Such a simple mental operation as the recognition of one of the less common flowers, say a particular orchid, will vary in duration according as we have or have not been recently forming an image of this flower.

The obvious explanation of this is that the mental image of an object bears a very close resemblance to the corresponding percept, differing from it, indeed, in degree only, that is to say, through the fact that it involves no actual sensation.