Just as we see that fallow land, if rich and fertile, teems with a hundred thousand wild and useless weeds, and that to set it to work we must subject it and sow it with certain seeds for our service; and as we see that women, all alone, produce shapeless masses and lumps of flesh, but that to create a good and natural offspring they must be made fertile with a different kind of seed; so it is with minds. Unless you keep them busy with some definite subject that will bridle and control them, they will throw themselves in disorder hither and yon in the vague field of imagination.

As when the light of waters in an urn,
Trembling, reflects the sun or moon, in turn
It flickers round the room, and darts its rays
Aloft, and on the panelled ceiling plays.

[Virgil]

And there is no mad or idle fancy that they will not bring forth in this agitation:

They form vain visions, like a sick man’s dreams.

[Horace]

The soul that has no fixed goal loses itself; for as they say, to be everywhere is to be nowhere:

He who dwells everywhere, Maximus, nowhere dwells.

[Martial]

Lately when I retired to my home, determined so far as possible to bother about nothing except spending the little life I have left in rest and privacy, it seemed to me I could do my mind no greater favor than to let it entertain itself in idleness and stay and settle in itself, which I hoped it might do more easily now, having become heavier and more mature with time. But I find —

Ever idle hours breed wandering thoughts

[Lucan]

— that, on the contrary, like a runaway horse, it gives itself a hundred times more trouble than it took for others, and gives birth to so many chimeras and fantastic monsters, one after another, without order or purpose, that in order to contemplate their strangeness and foolishness at my pleasure, I have begun to put them in writing, hoping in time to make even my mind ashamed of them.

..

— Montaigne
(Translated by Donald M. Frame)