Reason shows the means to an end; but if we are otherwise indifferent to the end, the reasonings fall inoperative on the mind.

Here then a _sentiment_ must display itself, a delight in the happiness of men, and a repugnance to what causes them misery.

Reason teaches the consequences of actions; Humanity or Benevolence is roused to make a distinction in favour of such as are beneficial.
He adduces a number of illustrations to show that reason alone is insufficient to make a moral sentiment.

He bids us examine Ingratitude, for instance; good offices bestowed on one side, ill-will on the other.
Reason might say, whether a certain action, say the gift of money, or an act of patronage, was for the good of the party receiving it, and whether the circumstances of the gift indicated a good intention on the part of the giver; it might also say, whether the actions of the person obliged were intentionally or consciously hurtful or wanting in esteem to the person obliging.

But when all this is made out by reason, there remains the sentiment of abhorrence, whose foundations must be in the emotional part of our nature, in our delight in manifested goodness, and our abhorrence of the opposite.
He refers to Beauty or Taste as a parallel case, where there may be an operation of the intellect to compute proportions, but where the elegance or beauty must arise in the region of feeling.