Introduction to Deontology: Kantian Ethics

Author: Andrew Chapman
Category: Ethics
Word Count: 1000

Using the Fat Man


You are walking along a trolley track when you come upon five people tied to the tracks. You frantically try to untie them, and as you realize that you’re going to need help and some heavy-duty tools to get them free, a trolley comes ‘round the bend. You try to signal to the conductor, but to no avail. Then you see that if you pull a nearby lever, the trolley will be diverted onto a side track, sparing the five unfortunate tied-up souls. But alas! On the side track, you notice that one person is tied up. So your choice is this:

  1. Do not pull the lever, in which case the trolley will hit and kill five people. (Don’t Switch)
  2. Pull the lever, in which case the trolley will hit and kill one person. (Switch)

This is known in the ethics literature as the Trolley Problem.1 What is the morally best, or at least morally better option? Most people, when presented with this thought experiment, assert that Switch is morally better or even required. Why? The overwhelmingly popular answer is that we should try to save as many lives as possible and there’s a simple calculation to be run: Five is greater than one. Switch.

But now consider this modification to the original experiment:

All of the initial conditions from the original experiment are in place, however, this time, there is no switch, no side track. Instead, there is an overpass above and before the five tied up individuals upon which is precariously perched a corpulent individual. If you give the plump person a push, sending him tumbling down onto the track, his body will act as a brake for the trolley, sparing the five unfortunate tied-up souls. So now your choice is this:

  1. Do not push the fat man, in which case the trolley will hit and kill five people. (Don’t Push)
  2. Push the fat man, in which case the trolley will hit and kill one person. (Push)

This is known as the Fat Man Variant. What is the morally best or better option here? Most people, when presented with this thought experiment, assert that Don’t Push is morally better or even required. But notice that in both the original and the variant case, your choice is one life vs. five lives. If we were purely concerned with total lives saved, then Don’t Switch and Don’t Push should be morally equivalent and Switch and Push should be morally equivalent. But they don’t seem to be so-equivalent to many people.2

Kantian Deontology & the Categorical Imperative

Can we articulate a coherent moral theory that makes sense of these initially apparently inconsistent moral intuitions? One popular moral theory that denies that morality is solely about the consequences of our actions is known as Deontology.3 The most influential and widely adhered to version of Deontology was extensively laid out by Immanuel Kant (1724–1804).4, 5 Kant’s ethics, as well as the overall philosophical system in which it is embedded, is vast and incredibly difficult. However, one relatively simple concept lies at the center of his ethical system: The Categorical Imperative

According to Kant, persons are essentially rational creatures deserving of respect.6 It is this necessary rational nature that issues in what Kant calls The Categorical Imperative: the single controlling ethical norm, from which all particular ethical rules derive. It is an imperative in that it issues commands or rules to us. It is categorical in that we can’t opt out of it—we are ethically bound by it because of the sorts of beings we are. While there is only one Categorical Imperative, rather than many interrelated but also fundamental moral rules, in his various works, Kant expresses The Categorical Imperative in a number of equivalent and equally binding formulations.7 Here, we’ll look at the second formulation:

Act in such a way that you treat humanity, whether in your own person or in the person of any other, never merely as a means to an end, but always at the same time as an end.

What this means, in simple terms, is that we are never allowed to merely use people. People aren’t non-rational objects, undeserving of respect, and as such, to treat them as if they were trees or slugs would be to disregard evidence and reason, to act irrationally, and hence, according to Kant, to sin against the very thing that makes us persons.8

The second formulation of the Categorical Imperative can help us make sense of the trolley problem intuitions with which we started this essay. In the original thought experiment, while flipping the switch would kill someone, we are not thereby using the person we kill. His death is an unfortunate and unintended consequence of our attempt to save five lives. In the fat man variant, pushing the large man in front of the train does use him—as a mere sack of sinew and bones, no better than a rock. This difference, says the Kantian, accounts for our moral intuitions and the differing moral status of the two cases.

A Problem for Kant

While a number of worries have been raised for Kantian Deontology, I here will touch on the one that is the most worrisome to many ethicists. Kant’s moral theory seems inflexibly bound to rules and duties. However, many people think that even if there are general moral rules, there are times when it is permissible or obligatory to break these rules. Consider a case famously outlined by Kant: If it is immoral to lie (as Kant thinks it is), then even if a known murderer comes to your door asking for the whereabouts of an acquaintance (presumably to murder her), you are obligated to tell the murderer your acquaintance’s whereabouts, since it is always immoral for you to lie.


While it seems to many as though there is something essentially right with a Kantian brand of ethics, i.e., that respect and duty are fundamentally important for our ethical lives, such a Kantian system does not come without its serious theoretical difficulties.


Thank you to Addison Ellis, Ryan Jenkins, and Duncan Purves for very helpful comments on an earlier version of this essay.

1This thought experiment is a work of hypothetical fantasy meant to stoke your ethical intuitions. Leave aside any implausibilities and notice that this situation is at least possible. Also, assume that all of the tied-up persons are morally equivalent, e.g., it’s not the case that one is a murderer and another is about to cure cancer. Finally, assume that the choices I’ve outlined are the only possible choices. For the original presentation of the trolley problem in the literature, see Foot (1978). For extensive analysis, see Thomson (1976).

2Notice that the Consequentialist should claim that since the consequences in terms of lives saved are the same in Don’t Switch and Don’t Push and in Switch and Push, we have morally equivalent pairs. See Shane Gronholz’s 1000-Word Philosophy essay “Introduction to Consequentialism” for much more on this popular moral theory.

3The Greek root, deont—, means a certain sort of necessity and has been interpreted as duty or obligation. Accordingly, Deontology can be seen as an ethics of duty.

4See especially the Groundwork of the Metaphysics of Morals, the Metaphysics of Morals, and the Critique of Practical Reason, all in Kant (1999)).

5While we will here only look at the most famous deontological ethical system, there are many more systems, some of them modifications on Kant’s account, some of them explicitly non-Kantian. For a nice overview and taxonomy of such theories, see Alexander & Moore (2012).

6It is a mistake to read “persons” as Homo sapiens. In Kant’s ethical sense, any self-conscious, reasons-responsive, autonomous creature will count as a person in the relevant sense. Accordingly, some non-human animals, e.g., apes, might count as persons. Similarly, potential future artificially intelligent computer systems could count as persons. Finally, some Homo sapiens, e.g., the very young and the severely mentally enfeebled, will not count as persons in the relevant moral sense.

7While there are different formulations of the Categorical Imperative, there is only one Categorical Imperative itself. Exactly how and in what way the different formulations are supposed to be equivalent (e.g., logical equivalence, synonymy, etc.) is a matter of much debate.

8All proper persons are just like you in their rational capacities, and hence, in the respect they deserve. To treat them differently, all evidence to the contrary, is both to treat them incorrectly by denying them the very thing that makes them moral persons and to lie to yourself, to mis-use your own rationality. Thank you to R. Jenkins and D. Purves for pressing me on this point.


Alexander, Larry, and Michael Moore. “Deontological Ethics.” Stanford Encyclopedia of Philosophy. Stanford University, 21 Nov. 2007. 08 June 2014. <>.

Foot, Phillippa, and Doris Schroeder. “The Problem of Abortion and the Doctrine of Double Effect.” Critical Concepts in Philosophy. Ed. Ruth Chadwick. Vol. 2. New York: Routledge, 1978.

Kant, Immanuel. Practical Philosophy. Trans. Allen W. Wood. Cambridge, U.K.: Cambridge UP, 1999.

Thomson, Judith Jarvis. “Killing, Letting Die, and the Trolley Problem.”Monist 59.2 (1976): 204-17.

Andrew is a lecturer in philosophy at the University of Colorado, Boulder. He holds a PhD in philosophy from the University of Colorado, Boulder, an MA in philosophy from Northern Illinois University and a BA in philosophy and a BM in bassoon and sound recording technology from Ithaca College. He specializes in epistemology, metaethics, and the history of philosophy (especially Kant and the 20th Century Anglophone and Phenomenological traditions). When not philosophizing, Andrew is skiing, hiking, listening to great music, or playing the bassoon.