T 3.2.5.2

I say, first, that a promise is not intelligible naturally, nor antecedent to human conventions; and that a man, unacquainted with society, could never enter into any engagements with another, even tho' they could perceive each other's thoughts by intuition. If promises be natural and intelligible, there must be some act of the mind attending these words, I promise; and on this act of the mind must the obligation depend. Let us, therefore, run over all the faculties of the soul, and see which of them is exerted in our promises.