Here are some practices that help me:
1. I try to follow my ideas to their axiomatic bases. I know when I've reached the foundation of an idea when I get to "just because," "I simply believe it's true," "I just feel it's true," "I live under the assumption that it's true" or "I don't know." If I can logically work back up from that axiom (or a set of axioms) to my idea, then I'm being as rational as a human can be.
For instance: "That's my cat."
What do I mean by "my"? I mean that I own the cat, but in what sense do I own it? Perhaps I immediately get to "I don't know" or "I just feel as if I do." Then I can say, "I feel as if I own my cat, but I can't really explain what that means or why I feel that way." That's perfectly rational. It may not be explanatory, and it may be dissatisfying, but it's not irrational.
Or, perhaps I can say, "I'm using the word 'own' to mean that the cat lives with me, I take responsibility for its well being, I would try to stop someone else from taking it away from me, and I feel I have the right to do so."
Why do I feel I have the right? Maybe, again, I don't know or that's just the way I feel. Or maybe I can go down another level: "Because I was brought up with the idea of ownership. All my life, everyone around me has trained me to think of objects (and pretty much everything except other people) as having no owner, being owned by me, or being owned by someone else. When I was a child, I was punished if I took something that I and other people thought of as being owned by someone else, and I was also brought up to feel a sense of injustice if someone took something that I felt I owned. The legal and moral systems I live under assume this to be true."
Why was I brought up that way? Why does such a legal system exist? Etc. I just need to keep following these questions until I get to "I don't know" or "I simply assume it to be true."
In a perfect argument between two people--a logical argument, not an angry argument--they dig towards axioms together. If they get all the way to the bottom level and find they disagree, there's little they can do. An axiom by its nature can't be proven. But they'll be crystal clear on the locus of their disagreement. If they agree on axioms, then the disagreement must be further up, and it's probably an error in logic on one or both of their parts. At least one of them is not drawing a conclusion from its premises.
We often have to think quickly, so it's impossible to always (or even usually) delve down to axioms. But in quiet moments we can pause to do it, especially when we find ourselves making the same arguments (or espousing the same beliefs) over and over. How many of your core beliefs have you examined this way?
Sometimes, I pick one of my paragraphs at random and circle or boldface the unexamined claims in it. As rational as I think I am, there are usually quite a few:
"We often have to think quickly, so it's impossible to always (or even usually) delve down to axioms. But in quiet moments we can pause to do it, especially when we find ourselves making the same arguments (or espousing the same beliefs) over and over. How many of your core beliefs have you examined this way? "
What makes me think that we "often have to think quickly"? It seems obvious that we do, but can I support this with any sort of evidence? Is it true for all people? Is it more true in some situations than others? What sort of situations? What exactly do I mean by "quickly"? Quickly compared to what?
Can we pause to do it? Do we have volitional control over our pauses? And can we "especially" pause when we find ourselves repeating arguments, or is it harder to pause at times like that? What do I mean by "we." Am I just talking about myself? Lots of people? People who have been educated in a certain way? All people?
If you want to think rationally, it's really helpful to, when you can, unpack the assumptions behind your claims, but if you're new to this practice, you'll hit "I don't know" quickly and often. You'll hate it, because--like most people who are rationalists--you want to believe you have logical reasons for all your ideas. This hatred will make you want to stop digging. But keep at it, and your digging skills will get better over time. Step one is admitting you don't know when you don't know, even if that's most of the time.
Be especially skeptical of stock phrases. The "rational club" has lots of these, such as "I just believe in one less god than you do" and "extraordinary claims require extraordinary evidence." I'm not saying these phrases are wrong (or right). I'm saying they can easily become ossified in your mind, shutting off all further thought on the matter. What, by the way does "extraordinary evidence" mean? How does it differ from "ordinary evidence"? What is the metric for deciding if a claim is extraordinary or not? You may have good answers for those questions or you may be just belching out those phrases without giving them much though.
Next time you want to tell someone that extraordinary claims require extraordinary evidence, try, as an exercise, making the same point with different words. If you can do it easily, that's a good sign. It probably means you understand what the phrase really means.
2. I created rules to help me discuss things calmly and rationally. As social animals, we often learn and reason through discussions, but they can easily backfire, becoming passive-aggressive (or outright aggressive) and more about maintaining ego than about clear thinking. I have forbidden myself from doing that, and I've compiled a list of phrases and situations that tend to lead me down the wrong path.
I'm far from perfect. I break my own rules. But the rules give me a goal to shoot for and a way to analyze my failures.
Marcus's Rules Of Order For Himself (Which He Invites You To Follow)
3. I keep a failure diary. This is a list of my mistakes with the best analyses I can make of them. I've published parts of it online: Personal Failures
We mostly learn by trial-and-error, which means constantly failing and pushing past failure. Unfortunately, most cultures stigmatize failure and teach us to avoid and deny it. Our attempt to cover up and justify our mistakes often leads to irrationality.
See Marcus Geduld's answer to Why do we get frustrated when learning something?
4. I avoid disputing definitions. Make sure that in any argument you're making to yourself or someone else, you make a clear distinction between labels and the things being labeled. Huge oceans of irrationality occur when people muddle those two categories.
"I mostly like girls, but every once in a while, I feel attraction to another man. Am I straight, gay, or bisexual?"
What you are is a person who mostly likes girls, but every once in a while you feel attraction to a man.
If we call you straight, you will still be a person who mostly likes girls but who is every once in a while attracted to a man;
if we call you gay, you will still be a person who mostly likes girls but who is every once in a while attracted to a man;
and if we call you bisexual, guess what you'll be? Still a person who mostly likes girls but is every once in a while attracted to a man.
Labels change nothing about you.
So what do you mean when you say "Am I straight, gay, or bisexual?" You already know you're a person who is mostly attracted to girls but who is every once in a while attracted to a man. So what exactly are you asking?
It's not necessarily something nonsensical, but you'll help your rationality by being explicit. Do you mean, "Given my tastes, would most people label me as straight, gay, or bisexual?" Are you asking how psychologists will categorize you?
Try to get clear why you care about the label. Is it because you've noticed that gay and bisexual people get treated unfairly, and you want to brace yourself for that? Do you want to know how other people are likely to label you? That's not a foolish concern, but it's different from "Am I ...?" at least when that phrase is taken literally.
If, on the other hand, your goal is to figure out how to live your life--whether to date men, women, or both--your "Am I ...?" question is pointless. Because it won't add any new information to the fact that you're mostly attracted to girls but every once in a while attracted to man.
Pay special attention to claims that involve forms of "to be." He is gay. She doesn't believe in God. They are friends. These phrases aren't necessarily irrational, but they are caves where irrationality often lurks. If you're a materialist, then try to think of the Universe as a place where objects have traits and behaviors. And do what you can to translate your to-be claims into that framework.
An object has parts. An object does stuff. It doesn't make literal sense to say that an object is... Where does it keep its is-ness? "Is" is a shorthand we use to mean "has traits X, Y and Z" or "does things A, B, and C." Rationality is served when you're explicit about the has and the does.
You can cut through a lot of bullshit "philosophy" that way: one brick is not a pile; two bricks isn't a pile; a hundred bricks is a pile. When is the exact transition between not-pile and pile? If you drop "is a pile," this whole muddle vanishes. That space on the floor has three bricks in it; That other space, over there, has 57 bricks in it. That's what actually exists: spaces on the floor containing a certain number of bricks. "Pile" is a human-invented label, and we've never created a robust meaning for it.
When you say that Bob "is" gay, what do you mean? What traits or actions does that imply? That he has the mental trait of feeling attraction towards other men? Does he always have that feeling? Does he feel attraction towards other men when he's asleep, when he's working on a Calculus problem? When he's furious at his boss? Maybe when you say he "is" gay, you mean that at various times, throughout his life, he has felt attraction for other men. And you're predicting he will, at many times in the future, fell that attraction again.
(Again, using "is" can lead to faux paradoxes. Bob only likes men. Mike mostly likes men, but 2% of the time he's attracted to women. Are they both gay? Is just one of them gay? What actually exists is a guy who is only attracted to men and another guy who is mostly attracted to men. The labels we use to refer to them are arbitrary.)
I am not saying you shouldn't use phrases like "Bob is gay." They're both useful and unavoidable. I'm suggesting that, if you want to improve your rationality, you'll see such phrases as shorthands and, when you have the time, try to unpack them. When I say someone is gay, what traits and behaviors am I referring to?
5. I practice writing and speaking in e-prime. It's a constrained version of English in which you're not allowed to use any form of "to be." I spent a year forcing myself to write in e-prime, and it improved both my thinking and my writing. (I also resolved not to tell anyone I was doing it, which made me labor to make my e-prime prose sound natural.) When you drop "to be," you will discover many pockets of irrationality in your thinking. (When you're unable to drop it, you should ask why.)
(I allowed myself to use "to be" in quotations.)
As with any any new form of writing, e-prime will be hard for you at first, but you'll slowly get better at it and eventually it will feel natural to you.
6. I explain or teach my ideas to other people, and if someone doesn't understand all or part of my argument, I assume it's my fault. Perhaps a better way to say this is "take responsibility for it." This habit is so useful to me and such a great indicator of the clarity of my thoughts, I assume that if I can't explain something so that everyone understands it, I don't fully understand it either.
Of course, it's possible the other person is simply stupid or lacks context. But you'll do your rationality great favors if you assume there's some way you can make him understand, and he hasn't understood so far because you've failed to be clear. Even do this (especially do this) if he's the only person out of a hundred who doesn't get your point. Instead of asking, "Why is he so stupid?" ask "What am I doing wrong?"
People who misunderstand are invaluable. They are like missiles which target weaknesses in your arguments. Maybe weaknesses in thinking; maybe weaknesses in explaining. (Which is almost the same thing.)
The more stupid someone's objection seems, the more closely you should examine that part of your argument. It may be a case of genuine stupidity, or it may uncover bias on your part--bias that some aspect of your argument you're sure is clear is actually not.
7. I closely examine my "should" phrases. You may have heard of the chasm between "is" and "ought." You can't prove what someone should or shouldn't do by simply noting facts about the world. For instance, the fact that people suffer from racism does not mean you shouldn't be racist. To conclude that, you have to couple the fact with a value (e.g. suffering is bad.)
I am not urging you to avoid "should." (And I'm certainly not arguing in favor or racism.) I am suggesting that you zoom in on your should phrases. They are nooks where irrationality tends to hide. We all tend to glide from is to ought, and we tend to do it without thinking.
8. I especially avoid should-phrases without the context of a goal. There is nothing anyone simply should do. People should do things if their goal is to be morally correct or because it's practical given a particular goal. Make sure you clarify your context. "Should I use a hammer?" makes no sense. For what? To pound nails, yes. To write with, no.
"You should do your homework."
No, you should do your homework if you want to get good grades in school or if you want your parents to be proud of you.
9. I avoid agentless or passive writing and thinking. They are huge hotbeds of irrationality.
- "Is boring, stable happiness preferable to an interesting but potentially less happy life involving more emotional risk?"
Preferable to whom? Preferable for what? There's no such thing as unpreferableness that just floats around in the universe, disconnected from a person or people. Even if you think the "whom" is obvious, go ahead and state it. You may find a smidgen of irrationality has crept into your thinking.
- "Have we lost all sense of shame?"
Who is "we"? Americans? Young Americans? Young educated Americans? Europeans? World citizens? People in third-world countries? Me and my friends? Me?
Who is doing what to whom?
10. If I'm on a side of some issue (abortion, liberalism vs conservatism, atheism vs theism), I need to figure out what's wrong with my side. Any major debate--one that has involved millions of people for many years--is bound to be complex. But our attraction to binary opposites tends to blind us to irrationality.
I'm not saying be a relativist and or that all options are equally good or valid. I'm saying most good medicines have side effects. They're not simply good with absolutely no downsides.
If you're a liberal, what's wrong with liberalism? Even if it's better than the alternative, what are some bad thing that can happen if we have liberal governments in power? If you're a conservative, what's wrong with conservatism?
Be careful when answering that you don't give a left-handed insult like those people who, when asked about their weaknesses in job interviews, say, "I'm sometimes too diligent for my own good." And make sure you don't just talk about "bad liberals" or "bad conservatives." if you’re a liberal, ask yourself what problems liberalism creates when it’s well practiced? If you’re a conservative, as yourself what problems conservatism creates--regular conservatism—not as practiced by crazy extremists?
If you can't think of an answer or you have very little to say on the subject, maybe that's because your side is simply perfect. But be skeptical. It may also be the case that you're biased and blinded by one of the most powerful forces that affect our species: tribalism.