In such a world, killing a wizard is much like scratching his hand without his permission. Do we have strong ethical intuitions that Voldemort’s actions are evil in such a world, where killing Harry’s parents amounted to much ado about nothing? Even though our intuitions here suggest that wizard murder may not be such a big deal in that world, is there anything we can reasonably thereby conclude about murder in our Muggle world?

What is the point of this digression on thought experiments? It is due to my impression Zwolinski and Brennan believe they have traditional libertarians in a dilemma: “You SAY you believe in universal moral principles, but what if they led to THIS [followed by pictures of corpses piled high, burned out cities, children on the rack, etc.]? Would you still support them THEN?” But I think the dilemma is flimsier than they appreciate.

Here’s the problem: The economic counterfactual on which Brennan bases his thought experiment is not like “Imagine demand for X is inelastic rather than elastic”. It is more like, “Imagine demand curves slope upward rather than downward for all goods over all ranges. Then what?!” Such thought experiments, based on such a gross and fundamental divorce from all empirical economic knowledge we have ever experienced, give us little guidance, just as (even though biology, too, is an empirical science) a thought experiment involving the immortality of wizards would give us little insight into appropriate ethical action in our world.

The Star Trek thought experiment above works because, even though we know the fantastical future world of Star Trek is different in many unknowable ways from our world, we see the characters interacting in ways that seem meaningful and understandable to us. They form friendships, loving bonds, command loyalty and affection, develop competitive feelings, seek wisdom and advantage, act heroically and ignominiously, and so on, as we do. So we believe we have intuitions as to their motivations and an understanding of their goals and desires. It seems natural, in other words, to apply to them moral motivations and strictures we would apply to ourselves.

Of course, the more fantastical the world of Star Trek, the more reasonable it becomes to potentially question such judgments. The marriage practices and rituals on Vulcan (the Pon Farr of “Amok Time,” for example), are such that our moral intuitions with respect to marriage may well not apply to Mr. Spock. And, as many Star Trek fans with even a basic understanding of economics have pointed out over the years (see, for example, this essay by Ilya Somin,) it is very difficult to coherently makes sense of the economics of Star Trek spin‐​offs like The Next Generation, given the post‐​scarcity society created by replicators and the absence of money as a common medium of exchange. So a thought experiment asking about economics in a Star Trek context would be much more challenging and much less clearly applicable to current economic issues than Star Trek thought experiments about morality.

In a discussion of thought experiments in his Reasons and Persons, Darek Parfit made the following distinction (p 219): “[C]onsider certain imaginary cases…I distinguish two kinds of case. Some cases contravene the laws of nature. I call these deeply impossible. Other cases are merely technically impossible. Does it matter if some imagined case would never be possible? This depends entirely on our question…[D]epending on our question, impossibility may make some thought experiment irrelevant.”

Presumably no one, including Brennan and Zwolinski, really believes they know of any historical instance where what traditional libertarians desire has been closely approximated and yielded anything remotely close to the results of their thought experiments. Is it fair to describe their pure thought experiments as divorced from real world experience in a very fundamental way, deeply impossible in Parfit’s sense?

Consider some well known philosophical thought experiments and see how the Brennan thought experiment compares. Take, say, the uncle Sam thought experiment with which Michael Huemer begins The Problem of Political Authority or the transfused violinist in Judith Thomson’s famous essay “A Defense of Abortion”.

In Huemer’s thought experiment, we know all there is to know about Sam. We know what his actions are, which are very similar to actions taken by government agents. We understand his motivations, which match the motivations of government officials. We appreciate that in all aspects save one–he wears no uniform; he is NOT a government agent–his actions perfectly mimic those of government agents. Were Sam a government agent, his actions would be perfectly acceptable to us, perhaps even praise‐​worthy. But he is not a government agent, and we are appalled. Thus this thought experiment allows us to focus on the issue of political legitimacy.

Similarly, Thomson’s famous thought experiment creates a person made into a patient without her initial consent. This is done to save the life of another. The medical condition and treatment described are fictional, but the moral issue–may one be forced to help another live?–is clearly analogous to the question of abortion, and pushed people to recognize there was more at issue than “the fetus is innocent.” Thomson’s comatose violinist is innocent as well, but the thought experiment focuses our attention on whether mere innocence is sufficient to justify forcing help from others.

How does the Brennan thought experiment compare? Poorly, I hope to now show. I appreciate that Brennan’s general answer to people (especially non‐​philosophers) who reject his thought experiment, and by implication Zwolinski’s positive thesis, is that they do so because they are not sophisticated as to the workings of thought experiments in modern philosophy. (As he put it on the BHL blog on March 16, 2011, “It can prevent them from being able to do philosophy at even an introductory undergraduate level.”) But I demur from that view. Thought experiments can be well constructed or they can be poorly constructed. Good thought experiments make relatively clear and focused changes in well understood contexts. Huemer’s uncle Sam is an individual taking clear and specific actions that are universally recognized as acceptable if he is an agent of the government, but in the context of the thought experiment he is merely acting as an individual. Thomson’s violinist is helpless and innocent, like a fetus, and the question of whether or not someone can thus be forced to assist for 9 months is brought to the fore. The choices offered in Nozick’s description of Newcomb’s Problem –the contents of the boxes and the conditions by which the contents are determined–are clear and unambiguous, if perhaps confusingly self‐​referential.

What is offered in Brennan’s thought experiment is, instead, unclear and hopelessly confused. Why is 90% of the populace immiserated? Is trade impossible despite the claim of free markets? Is this a society where only a few have even minimal skills, a complete world of Zero Marginal Productivity (ZMP)? Do the 10% of the populace that are well off have no incentive to become even better off by mutually beneficial interactions with the 90%? Is this a world where the law of comparative advantage is not in play? Is this a world where trade is not mutually beneficial? A world where specialization of labor does not improve productivity? To highlight the problem: This is a literally unimaginable world.

One clear piece of evidence the Brennan thought experiment is not fruitful is that it is too easily multiplied.