A $280 college textbook busts budgets, but Harvard author Gregory Mankiw defends royalties. By Richard Read. The Oregonian/OregonLive. post 
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Asked in an email, "Would you ever write an open-source textbook? Why, or why not?" Mankiw responded:
"Let me fix that for you: Would you keep doing your job if you stopped being paid? Why or why not?"
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The economist's fix makes clear that he cannot conceive of any activity, and especially any activity that one might be good at, as being other than a transaction.
I've found otherwise.