What is the main idea

I think the main theme is The Benefit of Free Exchange. When the market is left to itself and exchanges are free, both sides benefit. Indeed, no one would enter into an exchange that comes at a loss to them. In foreign commerce, this means that imports and exports can both be very valuable to a society. One society's wealth does not have to come at the expense of another society. A society has more to gain if its trading partners are wealthy.