The trolley problem problem
Are thoughts experiments experiments at all? Or something else? And do they help us think clearly about ethics or not?
最近许多分析哲学的工作都寄希望于对假想案例的研究。从罗伯特•诺齐克（Robert Nozick）和德里克•帕菲特（Derek Parfit）等哲学家的开创性贡献开始，他们倡导使用思维实验——设计简单的假设场景以探索或论证某种伦理原则的观点。这些场景通常没有语境的来龙去脉，而且往往与形成和实践伦理人情的日常环境大相径庭。其中最著名（或说最臭名昭著）的是“电车难题”思维实验，指在造成最小伤亡的情况下，从失控的有轨电车（或火车）前拯救更多人的可行性，此外还有数千种不同思想实验，有些研究甚至包含数十个单独案例。
While thought experiments are as old as philosophy itself, the weight placed on them in recent philosophy is distinctive. Even when scenarios are highly unrealistic, judgments about them are thought to have wide-ranging implications for what should be done in the real world. The assumption is that, if you can show that a point of ethical principle holds in one artfully designed case, however bizarre, then this tells us something significant. Many non-philosophers baulk at this suggestion. Consider ‘The Violinist’, a much-discussed case from Judith Jarvis Thomson’s 1971 defence of abortion:
虽然思维实验与哲学本身一样古老，但近代哲学却赋予了它独特的价值。纵使假设场景毫不现实，对它们的论断也被认为对现实世界的处事原则有着广泛的影响。这预设了如果你能在一个精心设计的案例中证明道德原则的某一点是正确的，不管其多么离奇，它都告诉了我们一些重要的东西。许多非哲学家对这一建议不以为然。让我们来看看“小提琴家”思维实验，这是朱迪思•贾维斯•汤姆森（Judith Jarvis Thomson）1971年为堕胎辩护时提出的一个备受讨论的案例：
You wake up in the morning and find yourself back-to-back in bed with an unconscious violinist. A famous unconscious violinist. He has been found to have a fatal kidney ailment, and the Society of Music Lovers has canvassed all the available medical records and found that you alone have the right blood type to help. They have therefore kidnapped you, and last night the violinist’s circulatory system was plugged into yours, so that your kidneys can be used to extract poisons from his blood as well as your own. The director of the hospital now tells you: ‘Look, we’re sorry the Society of Music Lovers did this to you – we would never have permitted it if we had known. But still, they did it, and the violinist now is plugged into you. To unplug you would be to kill him. But never mind, it’s only for nine months. By then he will have recovered from his ailment, and can safely be unplugged from you.’
Readers are supposed to judge that the violinist, despite having as much right to life as anyone else, doesn’t thereby have the right to use the body and organs of someone who hasn’t consented to this – even if this is the only way for him to remain alive. This is supposed to imply that, even if it is admitted that the foetus has a right to life, it doesn’t yet follow that it has a right to the means to survive where that involves the use of an unconsenting other’s body.
From the perspective of philosophers, the point here is clear, even if Thomson’s conclusion is controversial. In the few instances I tried to use this thought experiment in teaching ethics to clinicians, they mostly found it a bad and confusing example. Their problem is that they know too much. For them, the example is physiologically and institutionally implausible, and problematically vague in relevant details of what happened and how. (Why does the Society of Music Lovers have access to confidential medical records? Is the operation supposed to have taken place in hospital, or do they have their own private operating facility?) Moreover, clinicians find this thought experiment bizarre in its complete lack of attention to other plausible real-world alternatives, such as dialysis or transplant. As a result, excellent clinicians might fail to even see the analogy with pregnancy, let alone find it helpful in their ethical reasoning about abortion.
Thought-experiment designers often attempt to finesse the problem through an omniscient authorial voice that, at a glance, takes in and relates events in their essentials. The voice is able to say clearly and concisely what each of the thought experiment’s actors is able to do, their psychological states and intentions. The authorial voice will often stipulate that choices must be made from a short predefined menu, with no ability to alter the terms of the problem. For example, the reader might be presented with only two choices, as in the classic trolley problem: pull a lever, or don’t pull it.
All this makes reasoning about thought experiments strikingly unlike good ethical reasoning about real-life cases. In real life, the skill and creativity in ethical thinking about complex cases are in finding the right way of framing the problem. Imaginative ethical thinkers look beyond the small menu of obvious options to uncover novel approaches that better allow competing values to be reconciled. The more contextual knowledge and experience a thinker has, the more they have to draw on in coming to a wise decision. Ethical thought experiments work best when those who read them are willing to go along with the arbitrary stipulations of the author.
What would the world need to be like for thought experiments to be a good way of making progress in ethics? I’ll canvass two suggestions: first that the thought experiment is a kind of scientific experiment, and second that it is an appeal to imagination. As we will see, on either reading, thought experiments are highly fallible, and we should be circumspect about taking them to provide insights into real-world ethical problems.
If thought experiments are – literally – experiments, this helps to explain how they might provide insights into the way the world is. But it would also mean that thought experiments would inherit the two methodological challenges that attend to experiments more generally, known as internal and external validity. Internal validity relates to the extent to which an experiment succeeds in providing an unbiased test of the variable or hypothesis in question. External validity relates to the extent to which the results in the controlled environment translate to other contexts, and in particular to our own. External validity is a major challenge, as the very features that make an environment controlled and suitable to obtain internal validity often make it problematically different from the uncontrolled environments in which interventions need to be applied.
There are significant challenges with both the internal and the external validity of thought experiments. It is useful to compare the kind of care with which medical researchers or psychologists design experiments – including validation of questionnaires, double-blinding of trials, placebo control, power calculations to determine the cohort size required and so on – with the typically rather more casual approach taken by philosophers. Until recently, there has been little systematic attempt within normative ethics to test variations of different phrasing of thought experiments, or to think about framing effects, or sample sizes; or the extent to which the results from the thought experiment are supposed to be universal or could be affected by variables such as gender, class or culture. A central ambiguity has been whether the implied readers of ethical thought experiments should be just anyone, or other philosophers; and, as a corollary, whether judgments elicited are supposed to be expert judgments, or the judgments of ordinary human beings. As the vast majority of ethical thought experiments in fact remain confined to academic journals, and are tested only informally on other philosophers, de facto they are tested only on those with expertise in the construction of ethical theories, rather than more generally representative samples or those with expertise in the contexts that the thought experiments purport to describe.
The problems of external validity are even greater. The crucial question is: even assuming that a thought experiment has internal validity, what follows from the validity of judgments in the world of the thought experiment for other cases? If you agree that it would be permissible to pull the lever in the original trolley problem, causing five people to be saved and one to die, there are a variety of inferences that could follow. At the most confined, we could take it that the result has implications only for cases involving runaway trains with particular switching arrangements. At the other end of the spectrum, we could take the result to have far-reaching implications about the permissibility of causing harm to some in the course of preventing harm to greater numbers of others.
Judges within the common law tradition face a structurally similar question when making a judgment. They need to supply reasoning to support their decision, parts of which can be filleted out as the ratio decidendi (reason for the decision) by future judges. The ratio gives the judge’s best approximation to the breadth of the precedent the case sets.
The broader the precedents that thought experiments can set, the more powerful they will be for ethical thinking. In turn, the breadth of the precedents that a thought experiment sets depends on the degree to which the controls in place in the thought experiment, which allow the particular hypothesis to be tested cleanly, imply or are compatible with the wider cogency of the resultant ethical principle. This is not straightforward, and is itself a frequent topic for contestation.
The deeper question about external validity is whether thought experiments give insights into a single fixed picture that can gradually be reconstructed, or whether even well-designed thought experiments inform something more fragmentary, changeable and plural. Societies differ greatly in features such as wealth, inequality, population size, ethnic, linguistic and religious diversity, technological advancement, economic structure, ease of communication and travel, and the ability to collect taxes and maintain order without violence. Moreover, societies are continually shifting in terms of these structural variables, and sometimes rapidly, for example through processes of industrialisation or transition away from communism. The COVID-19 outbreak has vividly displayed the ways in which social norms and structures are more malleable than we assume. It is implausible to think that the actual optimal policy prescriptions would be the same, regardless of the societal context.
Many philosophers nonetheless wish to say that the correct ethical principles are unchanging. However, even if this were true, I suspect the principles wouldn’t be specific enough to provide useful advice, and the real work of ethical thinking would be in interpreting or specifying these principles. Compare a case where you go to someone for advice, and it transpires that you got exactly the same advice as everyone else, regardless of the specifics of your position.
An alternate view of thought experiments would downplay their relationship to scientific experiments, and acknowledge that they are, as Daniel Dennett put it, ‘intuition pumps’: tools for persuasion via imaginative consideration of possibilities. Thinking of thought experiments as persuasive fictions wouldn’t obviate the problem of external validity, but might allow us to reframe it.
The idea that fictions can provide ethical insights seems correct; but it doesn’t follow that they do so reliably or in a way that allows ethical insights to be easily transported from one context to another. One important question is what the relationship is between a well-told story and one that is true, or ethically insightful. How things are presented in fiction is often simplified and distorted
Using fiction as a means for ethical reflection – whether in thought experiments or in novels – will tend to raise the same questions of experience, abstraction and ‘too much knowledge’ that we considered earlier in discussing Thomson’s violinist.
In some ways, this criticism is as old as philosophical reflection on art. In his Republic, Plato complained that poets knew nothing about the things they wrote about, whether war or shoemaking, but presented images that others equally as ignorant would find convincing.
Overall, ethical thought experiments are, at best, fallible ways of constructing simplified models that map rather imperfectly onto the world as we experience it, and can distort as much as they illuminate. So should we give up on them as sources of ethical insight?
Responsible thinking requires calibrating our levels of credence to the reliability of our intellectual tools. Clearly, ethical thought experiments are not particularly reliable tools. But that’s not to say that we have other, more reliable tools. Pre-theoretical ethical ‘common sense’ is subject to distortions brought by prejudice, power and many other factors, and the reason why we turn to philosophical ethics in the first place is that it’s unclear how to resolve competing ethical duties that arise at a pretheoretical level. Ethical thinking is hard, and even our best tools for doing it are not very good. Humility should be the watchword.