Who is the narrator, can she or he read minds, and, more importantly, can we trust her or him?
Third Person (Omniscient)
Adam Smith introduces himself in the first person in the introduction to The Wealth of Nations, but apart from that he writes in the third person and delivers his opinions as though they are pure facts. For example, he'll write a line like this:
Nations tolerably well advanced as to skill, dexterity, and judgment, in the application of labour, have followed very different plans in the general conduct or direction of it. (I.7)
And this is basically what he spends the entire book exploring – the ways that different countries try to make themselves rich, and why some methods work and others don't.
When you're telling people how to run their country and their economy, it's probably best that you don't begin every sentence with "I sorta feel like this is the right way to do it, but feel free to disagree." Instead, you want to sound omniscient and rational, so Smith makes sure to remove himself from his claims, making them sound more like facts than opinions.