An Enquiry Concerning The Principles Of Morals - pg. 75

The end of all moral speculations is to teach us our duty; and, by proper representations of the deformity of vice and beauty of virtue, beget correspondent habits, and engage us to avoid the one, and embrace the other. But is this ever to be expected from inferences and conclusions of the understanding, which of themselves have no hold of the affections, nor set in motion the active powers of men? They discover truths: But where the truths which they discover are indifferent, and beget no desire or aversion, they can have no influence on conduct and behavious. What is honourable, what is fair, what is becoming, what is noble, what is generous, takes possession of the heart, and animates us to embrace and maintain it. What is intelligible, what is evident, what is probable, what is true, procures only the cool assent of understanding; and gratifying a speculative curiosity, puts an end to our researches.