"What is the experience of the other people I'm interacting with?"
I have sometimes found empathy to be a complicated concept, but this question really cuts to the heart of it and causes your brain to model the situation from the other person's perspective and use that in your decision-making.
Intriguingly, this seems to even apply to interactions with my future selves. If I don't ask this question or one like it, I'm likely to write a massive todo list for myself—probably a completely impossible list, and delegate it to my tomorrow!self with very little thought. Then when that self encounters the list, it's so overwhelming and inconsiderate that I find it hard to deal with at all. If instead, I stop to wonder what the experience will be of my future self encountering the tasks I've delegated for it, I realize that it makes sense to prioritize much of that up front, and to frame the delegation process with more context around why my past self wanted the thing done. This is basically the difference between saying "Future self, do this" and "Future self, at the moment it seems to make sense to me that you do this stuff, for reasons X, Y, Z." or some similarly empathetic request... which is much less likely to produce e.g. reactance.
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See also: Boring Advice Repository, Solved Problems Repository, Grad Student Advice Repository, Useful Concepts Repository, Bad Concepts Repository
I just got back from the July CFAR workshop, where I was a guest instructor. One useful piece of rationality I started paying more attention to as a result of the workshop is the idea of useful questions to ask in various situations, particularly because I had been introduced to a new one:
"What skill am I actually training?"
This is a question that can be asked whenever you're practicing something, but more generally it can also be asked whenever you're doing something you do frequently, and it can help you notice when you're practicing a skill you weren't intending to train. Some examples of when to use this question:
Many of the lessons of the sequences can also be packaged as useful questions, like "what do I believe and why do I believe it?" and "what would I expect to see if this were true?"
I'd like to invite people to post other examples of useful questions in the comments, hopefully together with an explanation of why they're useful and some examples of when to use them. As usual, one useful question per comment for voting purposes.