According to extreme solipsism, you’re the only person who really exists.  Suppose this strange position were true.  What would it imply for the Law of Comparative Advantage?

Consider a standard textbook problem with two agents: you and me.  By hypothesis, I’m a mere illusion, but I’m still a useful resource.  Suppose that an hour of time yields the following output.

Wheat

Steel

You

10

1

Me

1

5

Now suppose that the wheat:steel price ratio is 1:1.  Ordinarily, we’d say that you could just trade with me, making us both better off.  But on the solipsistic assumption, you can correctly regard me as a mere tool for converting one unit of wheat into one unit of steel.  For all practical purposes, then, you can forget about my existence, and simply recalculate your own productivity:

Wheat

Steel

You

10

10

Now suppose I’m the solipsist, and you’re illusory – a mere tool for me to convert one unit of steel into one unit of wheat.  Then I can forget about your existence, and simply recalculate my own productivity:

Wheat

Steel

Me

5

5

Of course, solipsism is false.  But we two solipsists can still teach the world a lesson.  Namely: in a deep sense, trade increases productivity:

Without Trade

With Trade

Wheat

Steel

Wheat

Steel

You

10

1

10

10

Me

1

5

5

5

You might object, “We already know that trade increases productivity via shared knowledge, increasing returns, etc.”  But these are all empirical claims.  My point is stronger: Trade tautologically increase productivity.  And if a solipsist can see this, so should everyone.