It is impossible for the mind to fix itself steadily upon one idea for any considerable time
 David Hume, A Treatise of Human Nature (1738). copy citation

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Author David Hume
Source A Treatise of Human Nature
Topic mind time
Date 1738
Language English
Reference
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Weblink http://www.gutenberg.org/files/4705/4705-h/4705-h.htm

Context

“In order to this we must reflect on certain properties of human nature, which though they have a mighty influence on every operation both of the understanding and passions, are not commonly much insisted on by philosophers. The first of these is the association of ideas, which I have so often observed and explained. It is impossible for the mind to fix itself steadily upon one idea for any considerable time; nor can it by its utmost efforts ever arrive at such a constancy. But however changeable our thoughts may be, they are not entirely without rule and method in their changes. The rule, by which they proceed, is to pass from one object to what is resembling, contiguous to, or produced by it.” source