Suppose I lose my eye in a car accident for which you were at fault. In our legal order you must pay me for the loss of my eye. But you get it at a bargain. I can get only what the court, or the worker’s comp schedules, or the insurance company says is the going rate for an involuntarily transferred eye as long as it still leaves me with one good one. You do not have to pay me what I would have demanded had you bargained with me ahead of time for the right to take it (assuming for the sake of the hypothetical that I could legally agree to have my eye gouged out). But that is the problem with accidents. One does not usually set about to do them on purpose, and so all the bargaining must he done after the transfer has been effected and the damage is done. My eye in such a regime is cheap for the taking.
Compare, though, how much improved my bargaining position is in a talionic regime, and thus how much pricier my eye will be. The talion structures the bargaining situation to simulate the hypothetical bargain that would have been struck had I been able to set the price of my eye before you took it. It does this by a neat trick of substitution. Instead of receiving a price for the taking of my eye, I get to demand the price you will be willing to pay to keep yours. It is not so much that I think your eye substitutable for mine. It is that you do. You will in fact play the role of me valuing my eye before it was taken out, and the talion assumes that you will value yours as I would have valued mine. The talion works some quick magic: as soon as you take my eye, in that instant your eye becomes mine; I now possess the entitlement to it. And that entitlement is protected by a property rule. I get to set the price, and you will have to accede to my terms to keep me from extracting.
William Ian Miller, quoted in this fascinating post synthesizing his ideas on talionic legal systems (“an eye for an eye”).