13-1a3a2 A necessary condition for an exchange to take place is that the two goods have reverse valuations on the respective value scales of the two parties to the exchange

“Since it takes two to make a bargain, Jones and Smith will exchange units of X for units of Y until one of them reaches a point beyond which further exchange will lead to loss rather than profit.”

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• 13-1a3a2a The difference is implied in any exchange and division of labor
• 13-1a3a3 The equilibrium price implies no further changes (the so-called subjective improvements)
• 13-1a3a2.1 An exchange is positive-sum only subjectively