Read it through once

If a man persists to inquire why he ought to promote the happiness of mankind, he demands a mathematical or metaphysical reason for a moral action. The absurdity of this scepticism is more apparent, but not less real than the exacting a moral reason for a mathematical or metaphysical fact. If any person should refuse to admit that all the radii of a circle are of equal length, or that human actions are necessarily determined by motives, until it could be proved that these radii and these actions uniformly tended to the production of the greatest general good, who would not wonder at the unreasonable and capricious association of his ideas?