Julia Galef: Think Rationally via Bayes’ Rule | Big Think
Think Rationally via Bayes’ Rule
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Bayes’ Rule is a formalization of how to change your mind when you learn new information about the world or have new experiences.
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JULIA GALEF:
Julia Galef is a New York-based writer and public speaker specializing in science, rationality, and design. She serves on the board of directors of the New York City Skeptics, co-hosts their official podcast, Rationally Speaking, and co-writes the blog Rationally Speaking along with philosopher of science Massimo Pigliucci. She has moderated panel discussions at The Amazing Meeting and the Northeast Conference on Science and Skepticism, and gives frequent public lectures to organizations including the Center for Inquiry and the Secular Student Alliance. Julia received her B.A. in statistics from Columbia in 2005.
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TRANSCRIPT:
I’d like to introduce you to a particularly powerful paradigm for thinking called Bayes’ Rule. Back in the Second World War the then governor of California, Earl Warren, believed that Japanese Americans constituted a grave threat to our national security. And as he was testifying as much to Congress, someone brought up the fact that, you know, we haven’t seen any signs of subterfuge from the Japanese American community. And Warren responded that, “Ah, this makes me even more suspicious. This is an even more ominous sign because that indicates that they’re probably planning some major secret timed to attack á la Pearl Harbor. And this convinces me even more that the Japanese Americans are a threat.”
So this pattern of reasoning is what sustains most conspiracy theories. You see signs of a cover up – well, that just proves that I was right all along about the cover up. You don’t see signs of a cover up, well that just proves that the cover up runs even deeper than we previously suspected.
Bayes’ Rule is provably the best way to think about evidence. In other words, Bayes’ Rule is a formalization of how to change your mind when you learn new information about the world or have new experiences. And I don’t think that the math behind – the math of Bayes’ Rule is crucial to getting benefit out of it in your own reasoning or decision making. In fact, there are plenty of people who use Bayes’ Rule on a daily basis in their jobs – statisticians and scientists for example. But then when they leave the lab and go home, they think like non-Bayesians just like the rest of us.
So what’s really important is internalizing the intuitions behind Bayes’ Rule and some of the general reasoning principles that fall out of the math. And being able to use those principles in your own reasoning.
After you’ve been steeped in Bayes’ Rule for a little while, it starts to produce some fundamental changes to your thinking. For example, you become much more aware that your beliefs are grayscale, they’re not black and white. That you have levels of confidence in your beliefs about how the world works that are less than one hundred percent but greater than zero percent. And even more importantly, as you go through the world and encounter new ideas and new evidence, that level of confidence fluctuates as you encounter evidence for and against your beliefs.
Also I think that many people, certainly including myself, have this default way of approaching the world in which we have our preexisting beliefs and we go through the world and we pretty much stick to our beliefs unless we encounter evidence that’s so overwhelmingly inconsistent with our beliefs about the world that it forces us to change our minds and, you know, adopt a new theory of how the world works. And sometimes even then we don’t do it.
So the implicit question that I’m asking myself that people ask themselves as they go through the world is when I see new evidence, can this be explained with my theory. And if yes, then we stop there. But, after you’ve got some familiarity with Bayes’ Rule what you start doing is instead of stopping after asking yourself can this evidence be explained with my own pet theory, you also ask well, would it be explained better with some other theory or maybe just as well with some other theory. Is this actually evidence for my theory.
Produced/Directed by Jonathan Fowler and Dillon Fitton
@bigthink
February 11, 2026 at 5:16 pm
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@worlQuache
February 11, 2026 at 5:16 pm
That is that psychopathy in full effect. 😢 Calcified pineal gland. Tabula rasa. If only we had a Curriculum Of Common Knowledge that is STEAM rich and logic based to assist humanity to stop this insanity. 🤔😊🤗
@ronarprefect7709
February 11, 2026 at 5:16 pm
Of course the math is important-it is a mathematical rule. Using the two-event version iteratively is WRONG mathematically(past the first use on a single data point). You want the three-event version after that.
@beezee7691
February 11, 2026 at 5:16 pm
what if we exercise standing alone in the center of the dark universe and im telling the light from you candle will suffice; to your point, i guess it is in regards to …
@19E37-e3i
February 11, 2026 at 5:16 pm
This lady fell off the edge of the earth after publishing her book a few years back. No goodbyes. Just stopped posting. Now her online assets are slowly rotting with broken links and trolls. What happened?
@mosesesle3225
February 11, 2026 at 5:16 pm
Occam’s razor.
@adp217
February 11, 2026 at 5:16 pm
I’d like to think Julia did actually explain what Bayes rule is but was let down by some poor filmmaking collaborators who cut it out. Definitely needed a clear explanation.
@NOKEEZY
February 11, 2026 at 5:16 pm
Found my type of people in these comments 👐🏼
@alphazutn1274
February 11, 2026 at 5:16 pm
Share this video with Andrew Tate's D riders.
@Cratees
February 11, 2026 at 5:16 pm
why is she so magneticc ughhh
@JasonGoodfellow
February 11, 2026 at 5:16 pm
I heard this talk a few years ago and adopted it, along with Julia's 'Scout mindset'.
Then I encountered a number of Marxist concepts and became interested. Applying both Bayes and scout mindsets, Marxist theory hit nothing but the net. It comprehensively accounted for the lack of political will to solve social problems. It used new tools to make sense of complicated world systems (historical materialism and material dialectics). And then make predictions.
It understands mass psychology/consiousness. Understands politics AND economics (political economy).
And it's been around for over 150 years, strawmanned, hushed and twisted because it criticises the current system that is hostile to it, capitalism.
It has been shocking how well it explains the human world, in comparison to all other philosophy and ideologies.
Visit 'Second Thought' for simple, neat concepts.
@ConfidentlyUninformed
February 11, 2026 at 5:16 pm
These are certainly great rules and I know that both of my parents, as medical professionals, learned them in their education, but that certainly isn’t always very clear in private. About the rule of thinking itself, now consider “Sherlock Homes’ rules” (“The Science of deduction) where he first gathers all important evidence and then makes a theory or conclusion.
@tod7977
February 11, 2026 at 5:16 pm
In reality, what sustains conspiracy theories are the constant lies, coverups and conspiracies commited daily by those in power.
What sustains the belief that this is not so, and there must be a psychological explaination as to why people believe in conspiracies, besides the fact they happen daily, is a loose grip on reality, a low level of knowledge of related facts and a general lack of discernment.
In 2023, someone that calls another a 'conspiracy theorist' is what I would call a 'moron'.
@HellRehab7732
February 11, 2026 at 5:16 pm
I'm guessing maga republicans aren't Bayesians.
@PaulValickas
February 11, 2026 at 5:16 pm
This was bad explanation.
@musikinspace
February 11, 2026 at 5:16 pm
For the vast majority of people, beliefs and worldviews are like clothes, chosen so that they can look cool, fit in, or even enhance their brand. That was a major red pill for me. People usually say they base their beliefs on logic as just another accessory so that they can look cool. But they don't really care about it at all.
@viriathas9910
February 11, 2026 at 5:16 pm
This is kinda sad, I hope this was some marketing guy's experiment to show his boss that he deserves his paycheck. Otherwise, this is just sad.
@funshorts1248
February 11, 2026 at 5:16 pm
This is all great and awesome but then confirmation bias comes in and destroys everything in our daily thinking.
@uttaranghosal5129
February 11, 2026 at 5:16 pm
Good speak
@blameyourself4489
February 11, 2026 at 5:16 pm
This cannot work with people who are not rational. However, we all tend to believe we are rational. So, at what stage is information then right or wrong? The more I learn about these things, the less confident I turn in understanding myself. I think it's good as rule to believe that we all as humans to some point react irrational on most things which are rational.
@wiffleone
February 11, 2026 at 5:16 pm
How does apply to love?
@AntonioSilva-ld4dq
February 11, 2026 at 5:16 pm
been able to change our mind when we learn new things i think goes beyond the mind. Repetition helps. Is a chemical stuff also, i found that magnesium gaves me flexibility in my patterns of thinking so when i have new info and that should be deploy in new behaviour because i learn it recently i blame lack of magnesium. Soon im going to start my own experiment and im going to use higher doses of magnesium to change or improve it with new tricks and facts.
@jimmymac3907
February 11, 2026 at 5:16 pm
In economics, we learned about the sunk cost mentality of some people's inability to reassess their beliefs…in context, "I've already put this much time and money into this project, I can't quit now"….this also explains why people can't escape conspiracy theory traps.
@matend8125
February 11, 2026 at 5:16 pm
Religious people , watch and learn
@devonk5720
February 11, 2026 at 5:16 pm
All these trumptard Unpatriotic, because attacking your government is The Most unpatriotic thing you Can Do, need to follow this rule.
@tabishbadar1320
February 11, 2026 at 5:16 pm
To apply Baye’s rule, 1. Your prior belief must be realistic, and 2. The more the prior overlaps with the evidence, the outcome is more consistent. Prior belief is rather crucial than anything else in Baye’s rule. Now, if you believe that the Earth is flat a priori, then, no matter what evidence M. Kaku or N. Tyson provide you, you’ll never going to belief that the Earth is like 🌎.
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