On the source of the causes of doubt in moral questions

What Aristotle wrote is perfectly true, that certainty is not to be found in moral questions in the same degree as in mathematical science. This comes from the fact that mathematical science completely separates forms from substance, and that the forms themselves are generally such that between two of them there is no intermediate form, just as there is no mean between a straight and a curved line. In moral questions, on the contrary, even trifling circumstances alter the substance; and the forms, which are the subject of inquiry, are wont to have something intermediate, which is of such scope that it approaches now more closely to this, now to that, extreme.

Thus it comes about that, between what should be done and what it is wrong to do, there is a mean, that which is permissible; and this is now closer to the former, now to the latter. Hence there often comes a moment of doubt, just as when day passes into night, or when cold water slowly becomes warm.