“In order that this may be clearly understood, let us conceive a very simple thing. For instance, a stone receives from the impulsion of an external cause, a certain quantity of motion, by virtue of which it continues to move after the impulsion given by the external cause has ceased. The permanence of the stone’s motion is constrained, not necessary, because it must be defined by the impulsion of an external cause. What is true of the stone is true of any individual, however complicated its nature, or varied its functions, inasmuch as every individual thing is necessarily determined by some external cause to exist and operate in a fixed and determinate manner.
“Further conceive, I beg, that a stone, while continuing in motion, should be capable of thinking and knowing, that it is endeavouring, as far as it can, to continue to move. Such a stone, being conscious merely of its own endeavour and not at all indifferent, would believe itself to be completely free, and would think that it continued in motion solely because of its own wish. This is that human freedom, which all boast that they possess, and which consists solely in the fact, that men are conscious of their own desire, but are ignorant of the causes whereby that desire has been determined. Thus an infant believes that it desires milk freely; an angry child thinks he wishes freely for vengeance, a timid child thinks he wishes freely to run away. Again, a drunken man thinks, that from the free decision of his mind he speaks words, which afterwards, when sober, he would like to have left unsaid. So the delirious, the garrulous, and others of the same sort think that they act from the free decision of their mind, not that they are carried away by impulse. As this misconception is innate in all men, it is not easily conquered. For, although experience abundantly shows, that men can do anything rather than check their desires, and that very often, when a prey to conflicting emotions, they see the better course and follow the worse, they yet believe themselves to be free; because in some cases their desire for a thing is slight, and can easily be overruled by the recollection of something else, which is frequently present in the mind.”
Benedict de Spinoza, Letter to G.H. Schaller (1674).