It maintains that morality is determined by sentiment.
It defines virtue to be WHATEVER MENTAL ACTION OR QUALITY GIVES TO A SPECTATOR THE PLEASING SENTIMENT OF APPROBATION; and vice the contrary.
We then proceed to examine a plain matter of fact, to wit, what actions have this influence.

We consider all the circumstances in which these actions agree, and thence endeavour to extract some general observations with regard to these sentiments.

If you call this metaphysics, and find anything abstruse here, you need only conclude that your turn of mind is not suited to the moral sciences.
II.

When a man, at any time, deliberates concerning his own conduct (as, whether he had better, in a particular emergence, assist a brother or a benefactor), he must consider these separate relations, with all the circumstances and situations of the persons, in order to determine the superior duty and obligation; and in order to determine the proportion of lines in any triangle, it is necessary to examine the nature of that figure, and the relation which its several parts bear to each other.

But notwithstanding this appearing similarity in the two cases, there is, at bottom, an extreme difference between them.