P003 • Probabilistic Reasoning
Also known as: Conjunction Rule, Probability Conjunction
When a story gets more detailed, it feels more real -- but it actually becomes less probable. The Litany of Occam is a reminder that every additional condition you add to an explanation is another way for it to be wrong. It is more probable that A, than that A and B together. This is not a guideline; it is a mathematical fact about how probability works.
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P(A and B) <= P(A)
Our minds are drawn to rich, specific narratives. A detailed prediction feels more convincing than a vague one, because detail activates our sense of plausibility -- we can picture it, so it feels more likely. This is the representativeness heuristic at work, and it is usually helpful: specific, coherent stories often do reflect real patterns in the world. The failure mode is that we confuse feeling plausible with being probable. Every additional detail in a conjunction is another condition that must hold true, which means every added detail can only make the overall prediction less likely, never more. Without this litany, we are vulnerable to the most vivid and elaborate story in the room, which is often the least likely to be exactly right.
| When you are evaluating competing explanations and need a principled way to weigh simplicity against elaborateness |
| When a prediction or conspiracy theory feels compelling because of how many details fit together, and you need a reminder that fit is not the same as likelihood |
| When you catch yourself adding conditions to a theory to make it feel more complete, and you want to check whether those additions are coming from evidence or from narrative satisfaction |
| When someone else's argument gains persuasive force from its specificity, and you want to separate the vividness of the story from the strength of the evidence |
| You find that a more detailed prediction or explanation feels more convincing than a simpler one, and you cannot quite point to the extra evidence that justifies the extra detail |
| You are comparing two accounts of something and the one with more narrative richness feels more trustworthy, even though the simpler one covers the known facts just as well |
| You notice yourself building an elaborate theory and each new piece makes you feel more certain rather than less -- which is the opposite of what probability would suggest |
| Someone presents a conspiracy or complex explanation with many interlocking parts, and the intricacy itself feels like evidence of its truth |
| Treating the litany as a reason to always prefer the simplest explanation -- it is about probability, not about what is definitely true. Sometimes the detailed explanation is correct; it just needs proportionally more evidence. |
| Confusing this with a ban on complexity. The litany does not say complex things do not happen. It says complex claims need more support, not less. |
| Forgetting that dependent probabilities work differently than independent ones -- if A makes B very likely, then P(A and B) can be close to P(A). The litany is most powerful when the additional conditions are not strongly implied by the first. |
| Litany of Occam |
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| It is more probable that A, than that A and B. The probability of a conjunction (A and B both being true) can never exceed the probability of either conjunct alone. |
| A fundamental law of probability that guards against the conjunction fallacy. Adding details to a story can make it feel more plausible due to representativeness, but mathematically it can only make it less probable. Every additional requirement is another way for the prediction to fail. |
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