Settle, for sure and universally, what conduct will promote the happiness of a rational being.

No-one can compel me to be happy in accordance with his conception of the welfare of others, for each may seek his happiness in whatever way he sees fit, so long as he does not infringe upon the freedom of others to pursue a similar end which can be reconciled with the freedom of everyone else within a workable general law – i.e. he must accord to others the same right as he enjoys himself.

Happiness is a very pretty thing to feel, but very dry to talk about.

...[A]ll the elements which belong to the notion of happiness are altogether empirical, that is, they must be borrowed from experience, and nevertheless the idea of happiness requires an absolute whole, a maximum of welfare in my present and all future circumstances. Now it is impossible that the most clear-sighted and at the same time most powerful being (supposed finite) should frame to himself a definite conception of what he really wills in this. Does he will riches, how much anxiety, envy, and snares might he not thereby draw upon his shoulders? Does he will knowledge and discernment, perhaps it might prove to be only an eye so much the sharper to show him so much the more fearfully the evils that are now concealed from him and that cannot be avoided, or to impose more wants on his desires, which already give him concern enough. Would he have long life? Who guarantees to him that it would not be a long misery? Would he at least have health? How often has uneasiness of the body restrained from excesses into which perfect health would have allowed one to fall, and so on? In short, he is unable, on any principle, to determine with certainty what would make him truly happy; because to do so he would need to be omniscient.

Happiness is not an ideal of reason but of imagination

Morality is not really the doctrine of how to make ourselves happy but of how we are to be worthy of happiness