… is from page 356 of the late Paul Heyne ‘s profound 1981 article “Measures of Wealth and Assumptions of Right: An Inquiry” as it is reprinted in the 2008 collection of Heyne’s writings, “Are Economists Basically Immoral?” and Other Essays on Economics, Ethics, and Religion  (Geoffrey Brennan and A.M.C. Waterman, eds.) (original emphasis):
Suppose an armed robber confronts you with the choice, “Your money or your life.” We would say that you are not engaging in a voluntary exchange when you hand him the money in your wallet. We call that coercion, I suggest, because the robber induces you to do what he wants by threatening to reduce your options.
Contrast this situation with the one created by a cab driver who won’t give you a ride unless you give him your money. If you choose in this case to turn over the contents of your wallet, we would all agree that you are engaging in a voluntary transaction. The difference between the robber and the cab driver is that the cab driver induced you to do what he wanted by offering to extend your options.
It follows that we must know who has what options to begin with if we are to be able to distinguish a voluntary from an involuntary transaction. Is someone who induces you to do what he wants by threatening not to give you something you would like to have coercing you? He is not coercing you if what is in his power to bestow or withhold is in fact his and not yours. If the good is by right yours already, then he is coercing you.
DBx: It makes all the difference whether the other person’s hand is offering or grabbing. A hand that offers is a hand that promotes positive-sum mutual gain – gain to both. A hand that grabs is a hand that promotes negative-sum one-sided gain – gain to the grabber and loss for the grabbee. (Such grabbing is negative sum for two reasons. First, unlike offerers, grabbers need not produce anything in order to get what they want. Second, the risk of having their stuff grabbed leads grabbees to produce less stuff to be grabbed, in part because grabbees are incited to spend more time avoiding the grabbers.)
As Heyne notes, however, before we can determine whether what is going on in any particular case is an “offer” or a “grab,” we must first know what the property-rights arrangement is. But the necessity of – and sometimes difficulty in making – this foundational determination ought not blind us to the central importance of the distinction between what I call here “offerers” and “grabbers.”