When describing the virtues of man, David Hume divides them into two catergories: natural and artificial. Many arguments have been presented as to why he felt compelled to do this and in this essay I shall be examining why Hume decided upon these two distinct categories of virtue, and his justification for doing so.
When presenting his moral philosophy in 'A Treatise of Human Nature', Hume examines how and why humans act and live as moral beings. He tries to find out the extent of our morality, our rational self and how much we are influenced by our passions. When looking at our moral behaviour, Hume concludes that when making moral judgements, there are no intrinsically right or wrong actions, because "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."  This maxim seems a rather curious starting point to establish a rule for virtue, as I would argue that it could be quite conceivable to witness a virtuous action, yet with immoral intentions or motivation. I think it is also not a far stretch to imagine a virtuous intention being either not carried out, or resulting in a less than virtuous act. I would argue that both the act, and the motive can be judged as either virtuous or not, but that it is the motive that truly decides the virtue of an act.
Natural virtues have been described as "the basis of actions we morally approve of and are not dependent on any human convention."  This I think is a fairly clear definition of what Hume would consider a natural virtue. There are most commonly identified, four types of natural virtue, or sources of "personal merit"  . These are: Virtues considered "useful to society"  , such as "fidelity, justice, veracity, integrity,"  'social virtues' including "meekness, beneficence, charity, generosity, clemency, moderation, equity."  Other virtues considered natural are those useful to the agent himself, these include "prudence, temperance, frugality, industry, assiduity, enterprise, dexterity...generosity and humanity,"  any virtues "intrinsically pleasing to those who encounter or consider them, such as wit, eloquence, ingenuity, decency and decorum,"  and "those intrinsically pleasing to the agent himself, including cheerfulness, serenity and contentment"  are all considered natural virtues as well. Hume believes that the strongest of these natural virtues is benevolence, and that many other natural virtues stem from benevolence.  The reason Hume picks benevolence as the prime natural virtue of man, is because our benevolence and sympathy are what Hume believes allowed us to form a civilised society of law and justice. Hume seems to place the motive for natural virtues as an intrinsic motivation of humans to act in a kind manner. One justification that Hume gives for his natural virtues is that they are an instinctive part of human nature, so much so that they can be found in all nations in the world, as well as in all ages of humans. 
Another reason Hume gives for separating natural virtues from artificial ones is that the rules of natural virtues are not very complex. For example, when talking about the artificial nature of justice, Hume writes that it could not have possibly come from nature because the rules are "too numerous to have proceeded from nature."  He therefore believes that natural virtues have a much simpler, primeval sense of order to them, compared to the complexities of artificial virtues. The basis for natural virtues I think is the animalistic instincts that humans developed when having to survive without society. The natural morals that animals show such as love for their offspring and mate, but fear and distrust of outsiders I think is something that has shaped the natural virtues that humans possess. Hume writes about how the congenial nature of society may be more artificial than natural: "we perceive, that the generosity of men is very limited, and that it seldom extends beyond their friends and family, or, at most, beyond their native country. Being thus acquainted with the nature of man, we expect not any impossibilities from him; but confine our view to that narrow circle, in which any person moves, in order to form a judgement of his moral character."  I think that Hume is distinguishing that in terms of natural virtue, one will always treat their close friends and family with more compassion than a stranger, and that is why it is considered natural, because its germination was in nature.
Artificial virtues, on the other hand, are virtues that are not intrinsic part of human nature, but arise as a result of an enlightened and co-operative human society. Hume picks out a few key artificial virtues; these are "justice, keeping promises, allegiance and chastity."  Artificial virtues rise out of the conventions of civilised society. They are important for the preservation of social cooperation and the rule of law. Hume defines these virtues as artificial, because although they are not intrinsically virtuous, but instead are "purposely contriv'd and directed to a certain end."  For example, following the rules of society may not be beneficial at first, but can benefit the individual in the long run "observance of these rules, both in general, and in every particular instance, they are at first mov'd only by a regard to interest; and this motive, on the first formation of society, is sufficiently strong and forcible." 
The motivations for artificial virtues are less instinctive and more contrived than their natural counterparts. Instead of the natural inclination towards benevolence and generosity, Hume argues that artificial virtues demonstrate the more rational nature of man. The rules are complex and have "no marks of a natural origin", because they are "too numerous to have proceeded from nature."  The circumstances for which artificial virtues can occur are much more specific than for natural virtues. For instance justice depends on the artificial circumstances of scarcity and limited propensity for benevolence and cooperation. But artificial virtues are not arbitrary under such condition. "Justice takes its rise from human conventions". Justice rises out of the "scarcity (of nature) in comparison of the wants and desires of men". If nature provided enough for everyone and if man were not so selfish then justice would not exist. 3.2.2 SB 494
Hume makes He describes justice as "highly conductive, or indeed absolutely requisite, both to the support of society, and the well-being of every individual."  Hume argues that justice is only needed when social cooperation is required.