P17: A Desire which arises from a true knowledge of good and evil, insofar as this concerns contingent things, can be restrained much more easily still by a Desire for things which are present.

Dem.: This Proposition is demonstrated in the same way as the preceding one, from P12 C.

Schol.: With this I believe I have shown the cause why men are moved more by opinion than by true reason, and why the true knowledge of good and evil arouses disturbances of the mind, and often yields to lust of every kind. Hence that verse of the Poet: … video meliora, proboque, deteriora sequor …

Translation

I see better things and approve them, I follow worse things.

Ecclesiastes also seems to have had the same thing in mind when he said: “He who increases knowledge increases sorrow.” I do not say these things in order to infer that it is better to be ignorant than to know, or that there is no difference between the fool and the man who understands when it comes to moderating the affects. My reason, rather, is that it is necessary to come to know both our nature’s power and its lack of power, so that we can determine what reason can do in moderating the affects, and what it cannot do. I said that in this part I would treat only of man’s lack of power. For I have decided to treat Reason’s power over the affects separately.