T 3.2.5.4

All morality depends upon our sentiments; and when any action, or quality of the mind, pleases us after a certain manner, we say it is virtuous; and when the neglect, or non-performance of it, displeases us after a like manner, we say that we lie under an obligation to perform it. A change of the obligation supposes a change of the sentiment; and a creation of a new obligation supposes some new sentiment to arise. But 'tis certain we can naturally no more change our own sentiments, than the motions of the heavens; nor by a single act of our will, that is, by a promise, render any action agreeable or disagreeable, moral or immoral; which, without that act, wou'd have produc'd contrary impressions, or have been endow'd with different qualities. It wou'd be absurd, therefore, to will any new obligation, that is, any new sentiment of pain or pleasure; nor is it possible, that men cou'd naturally fall into so gross an absurdity. A promise, therefore, is naturally something altogether unintelligible, nor is there any act of the mind belonging to it[77].