One of the fundamental ideas of economics is Pareto efficiency. Wikipedia has a good summary. This will suffice:
"Given a set of alternative allocations and a set of individuals, a movement from one allocation to another that can make at least one individual better off, without making any other individual worse off..."
The key words are "better" and "worse".
David Ricardo is responsible for another fundamental idea, comparative advantage.
"...it can be beneficial for two parties (countries, regions, individuals and so on) to trade if one has a lower relative cost of producing some good. What matters is not the absolute cost of production but the opportunity cost, which measures how much production of one good is reduced to produce one more unit of the other good."
The key word in this case is "beneficial".
I propose to give some little scenarios and see where they lead.
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