T 3.1.1.25

But to chuse an instance, still more resembling; I would fain ask any one, why incest in the human species is criminal, and why the very same action, and the same relations in animals have not the smallest moral turpitude and deformity? If it be answer'd, that this action is innocent in animals, because they have not reason sufficient to discover its turpitude; but that man, being endow'd with that faculty, which ought to restrain him to his duty, the same action instantly becomes criminal to him; should this be said, I would reply, that this is evidently arguing in a circle. For before reason can perceive this turpitude, the turpitude must exist; and consequently is independent of the decisions of our reason, and is their object more properly than their effect. Ac|cording to this system, then, every animal, that has sense, and appetite, and will; that is, every animal must be susceptible of all the same virtues and vices, for which we ascribe praise and blame to human creatures. All the difference is, that our superior reason may serve to discover the vice or virtue, and by that means may augment the blame or praise: But still this discovery supposes a separate being in these moral distinctions, and a being, which depends only on the will and appetite, and which, both in thought and reality, may be distinguish'd from the reason. Animals are susceptible of the same relations, with respect to each other, as the human species, and therefore wou'd also be susceptible of the same morality, if the essence of morality consisted in these relations. Their want of a sufficient degree of reason may hinder them from perceiving the duties and obligations of morality, but can never hinder these duties from existing; since they must antecedently exist, in order to their being perceiv'd. Reason must find them, and can never produce them. This argument deserves to be weigh'd, as being, in my opinion, entirely decisive.