Ethics and morality are hefty topics but, luckily, we have David Hume (Scotsman, philosopher, historian, and all-around fountain of knowledge) on hand to guide us.
Philosophy can sometimes be daunting, but Hume tries to make it something readers can relate to. As Hume said himself, his previous works maybe weren't the easiest reads. That's why he keeps the readers in mind and uses lots of examples that we can recognize and understand. For example, he points out that
The tortoise, according to the fable, by his perseverance, gained the race of the hare, though possessed of much superior swiftness. (VI.I.10)
Using fables such as this is a great way of getting the message across without becoming caught up in complex, abstract theories (which is a guaranteed ticket to snoozeville).
The Enquiry as a whole centers on Hume's thoughts about morality. Hume wastes no time in letting us know that, in his view, a lot of philosophers have been way off-base in their writings on this topic. So, as well as making the Enquiry accessible, another of his aims is to clear up the mess that other theorists have made. This piece is essentially one big "Hold up—wait a minute" from Hume to the other philosophers of his day.
One criticism Hume has of other philosophers is that they sometimes seem pretty vague and detached from reality. As Hume puts it, they've "commonly endeavoured to account for these distinctions by metaphysical reasonings, and by deductions from the most abstract principles of the understanding" (I.4). Put it another way—these guys have lost the plot.
We need to remember that Hume's an empiricist, which means that he's focused on the role that the senses play in gaining knowledge. This goes against rationalism, which argues that we obtain knowledge independently of the senses (head to this site for more on the empiricism/rationalism debate).
This doesn't mean that Hume is down on reason. In a trial, for instance, Hume recognizes that "the first object of the prisoner is to disprove the facts alleged, and deny the actions imputed to him […] It is confessedly by deductions of the understanding, that the first point is ascertained" (I.5). However, he believes that it's only when sentiment emerges that morality comes alive.
Summing up the difference between reason and sentiment, Hume explains that "one discovers objects as they really stand in nature, without addition and diminution: the other has a productive faculty, and gilding or staining all natural objects with the colours, borrowed from internal sentiment, raises in a manner a new creation" (I.21). If reason is the canvas, then sentiment's the paintbrush.
Another thing that Hume criticizes is the extent to which some modern thinkers rely on theology. It's as though they start out with certain religious views then force everything else to agree with them. Hume makes his feelings clear when he argues that theology "admits of no terms of composition, but bends every branch of knowledge to its own purpose, without much regard to the phenomena of nature, or to the unbiassed sentiments of the mind" (AIV.21). For Hume, these guys are closing philosophy off rather than opening it up.
Hume has his own firm ideas about morality, of course, but he stresses that he's arrived at them "from reasoning and argument" (XI.I.13) rather than arrogance, impulsiveness, or prejudice. He makes sure we know the hows and whys of his theory so that we don't just think, "wow, this guy's full of himself."
Despite his convictions, Hume reveals that even he has the odd moment of doubt. He sees it as so obvious that morality involves reason and sentiment that he can't believe some folk don't buy this. As he confides, this causes him to "fall back into diffidence and scepticism, and suspect that an hypothesis, so obvious, had it been a true one, would, long ere now, have been received by the unanimous suffrage and consent of mankind" (IX.I.13). He quickly recovers from this wobble, though, and we don't see this as a flaw—if anything, it's like, "phew, this guy's only human after all."
Hume's pretty consistent throughout the Enquiry, and it's not difficult to follow his train of thought. Luckily, he stays on topic and keeps returning to his key points and arguments rather than going off on flights of fancy.
Hume certainly gives us plenty of examples to back up his theory. These examples are often taken from ancient Greece and Rome but, even if we don't know the backstory, we get what Hume's saying. This is all the more impressive when we remember that Hume wrote this in the eighteenth century. Sure, some of the language may be kind of dated (when did you last use the word "knave" in conversation?), but the main points still hold up. Because of this, a text that could've gone the way of the dinosaurs has managed to stay relevant. So, major props to Hume on that one.
One of Hume's strengths is showing how ideas about virtue can change. There's some stuff like compassion and reliability that are always in fashion, but other stuff can depend on the time and/or place we're dealing with. Hume notes, for example:
In ancient times, bodily strength and dexterity, being of greater use and importance in war, was also much more esteemed and valued, than at present. (VI.II.4)
These guys had to be pretty buff to be successful.
Another of Hume's aims is to show that human beings aren't totally selfish. Sure, we have a sense of self-interest, but we have a natural sense of humanity and sympathy, too. There may be the odd person who gets an "F" grade in these areas, but this doesn't mean that we should all be tarred with the same brush.
Observing that we're capable of feeling emotions about stuff that happened a long time ago or somewhere far away, Hume concludes:
The voice of nature and experience seems plainly to oppose the selfish theory. (V.I.6)
As well as the natural virtues, there are other virtues that Hume sees as having been created for a purpose. Justice is a topic that Hume spends a lot of time on, his argument being that systems of justice have been set up because they're necessary and useful.
Hume makes it clear that usefulness is one of the main things that makes us think, "yep, this is a virtue." The same goes for agreeableness. Throughout the piece, these are the two qualities that Hume keeps coming back to. Going into fanboy mode, Hume raves:
What praise is implied in the simple epithet useful! What reproach in the contrary! (II.II.8)
This brings us to one of Hume's most memorable claims: that "monkish virtues" like fasting, solitude, and self-denial shouldn't be celebrated. Are they useful? Are they agreeable? For Hume, the answer to both is a big fat "no." Some folks (especially religious types) wouldn't agree with him. What if someone finds solitude agreeable? But Hume doesn't entertain any ideas of this kind. Nope, he believes that these supposed virtues "stupefy the understanding and harden the heart, obscure the fancy and sour the temper" (XI.I.3). As you can see, Hume isn't a guy who holds back.