What is the Most Important Question in Ethics?

It is also important to recognize that there are ethical reasons for thinking in certain ways, or with certain concepts. As those working in ‘conceptual engineering’ have brought out especially clearly in recent years, the use of certain concepts or lines of thought is itself open to ethical assessment (consider e.g. Sally Haslanger’s work on race and gender).
With these cases, Parfit illustrates well how the ‘I’ question isn’t suited to many significant collective action problems. Consider climate change. Many people think they might have to do something to prevent a climate disaster. But – unless they have significant influence – they can plausibly conclude: ‘Whether this disaster happens or not won’t depend at all on what I do. So I’ll keep on living in the same way’. And, of course, if large numbers of people do this, then a climate disaster may occur which is worse for all of them, and every other living person, than if they’d all changed their behaviour.
The most important question in ethics, then, is not ‘How should one live?’, but ‘How should we live?’. And if you find yourself asking the question ‘How should I live?’, the answer is: ‘Ask yourself how we should live, and then take it from there’.  The Bad Old Days.
Here, if a torturer asks the ‘I’ question, they might plausibly answer: ‘I like torturing, and I’m not doing any harm, so I’ll keep on torturing’. Here the question, ‘What should I do?’, works well. If any torturer asks it, at the end of a day, the answer will be: ‘Stop torturing!’. But now consider another of Parfit’s cases: by Roger Crisp
That is usually understood as equivalent to: ‘How should I live?’. If so, then I’m not sure that this is the most important ethical question. Consider the following case from Derek Parfit’s Reasons and Persons: A thousand torturers have a thousand victims. At the start of each day, each of the victims is already feeling mild pain. Each of the torturers turns a switch a thousand times on some instrument. Each turning of a switch affects some victim’s pain in a way that is imperceptible. But, after each torturer has turned his switch a thousand times, he has inflicted severe pain on his victim.
In The Bad Old Days, each torturer inflicted severe pain on one victim. Things have now changed. Each of the thousand torturers presses a button, thereby turning the switch once on each of the thousand instruments. The victims suffer the same severe pain. But none of the torturers makes any victim’s pain perceptibly worse.
The Harmless Torturers
It’s often been said (including by Socrates) that the most important, ultimate, or fundamental question in ethics is: ‘How should one live?’.
Our culture is individualistic: the ‘I’ question comes naturally to us. Other cultures are more collective: in these cultures, practical reasoning begins from, or at least takes very seriously, the question: ‘What should we do?’. There is no reason to think that, in itself, the individualist question is the place to start ethical thinking.