Hume declares that there is a causal link between the passions felt by a man and the acts done by him. Human acts are the outcome of whatever causes, including passions, are influential, and that there is no mysterious change - called "free will" - between moral causes and moral effects.
Reason is not sufficient to distinguish virtue and vice, "moral sense" is the final arbiter
Hume's basic moral question: "Why any action or sentiment upon the general view or survey, gives a certain satisfaction or uneasiness?" This comes from his view that virtue is distinguished by the pleasure, vice by the pain, that any action, sentiment or character gives us by the mere view and contemplation.
No action can be virtuous, or morally good, unless there be in human nature some motive to produce it, distinct from the sense of morality.
The sense of justice and injustice is not derived from nature, but arises artificially, though necessarily from education, and human conventions. (p. 535.) ... Mankind is an inventive species; and where an invention is obvious and absolutely necessary, it may as properly be said to be natural as any thing that proceeds immediately from original principles, without the intervention of thought or reflection. Though the rules of justice be artificial, they are not arbitrary. They are artificial in that they need to be invented. The rules of justice provide the advantages of additional force, ability, and security. Our strongest attention is confined to ourselves; our next is extended to our relations and acquaintance; and 'tis only the weakest which reaches to strangers and indifferent persons.
Alone, advantage is not sufficient to cause us to create and follow the rules of justice. People must be sensible of these advantages. Study and reflection are not sufficient. They have to be combined with "the natural appetite betwixt the sexes, which unites them together, and preserves their union, till a new tie takes place in their concern for their common offspring. This new concern becomes also a principle of union betwixt the parents and offspring, and forms a more numerous society; where the parents govern by the advantage of their superior strength and wisdom, and at the same time are restrained in the exercise of their authority by that natural affection, which they bear their children. ... It is only from the selfishness and confined generousity of men, along with the scanty provision nature has made for his wants, that justice derives its origin.
A man naturally loves his children better than his nephews, his nephews better than his cousins, his cousins better than strangers, where everthing else is equal. Our sense of duty always follows the common and natural course of our passions.
Reference:
Hume, D. A Treatise of Human Nature, Penguin Classics (London, UK:1987). First printed in 1739.