Every man is conscious of a succession of thoughts which pass in his mind while he is awake, even when they are not excited by external objects.
The mind on this account may be compared to liquor in the state of fermentation. When it is not in this state, being once at rest, it remains at rest, until it is moved by some external impulse. But, in the state of fermentation, it has some cause of motion in itself, which, even if there is no impulse from without, suffers it not to be at rest a moment, but produces a constant motion and ebullition, while it continues to ferment.
Thomas Reid, Essays on the Intellectual Powers of Man, Essay IV, Chapter IV.