Imagination is strong in a man when that particular function of the brain which enables him to observe is roused to activity without any necessary excitement of the senses. Accordingly, we find that imagination is active just in proportion as our senses are not excited by external objects. A long period of solitude, whether in prison or in a sick room; quiet, twilight, darkness — these are the things that promote its activity; and under their influence it comes into play of itself. On the other hand, when a great deal of material is presented to our faculties of observation, as happens on a journey, or in the hurly-burly of the world, or, again, in broad daylight, the imagination is idle, and, even though call may be made upon it, refuses to become active, as though it understood that that was not its proper time.

However, if the imagination is to yield any real product, it must have received a great deal of material from the external world. This is the only way in which its storehouse can be filled. The phantasy is nourished much in the same way as the body, which is least capable of any work and enjoys doing nothing just in the very moment when it receives its food which it has to digest. And yet it is to this very food that it owes the power which it afterwards puts forth at the right time.

Schopenhauer, Studies in Pessimism