The precepts in this group are the working toolkit of reasoning: razors for choosing between explanations, procedures for estimating and planning, rules for disagreeing productively, and a few ethical principles old enough to have been argued over for centuries. Their value lies less in any single rule than in the way they correct each other.
Razors for choosing explanations
The razors each tell you which explanation to try first. Occam's Razor prefers the simplest adequate one. The Zebra Rule, from medical teaching, says to think of horses before zebras when you hear hoofbeats. Hanlon's Razor prefers error to malice, and Hitchens's Razor says what is asserted without evidence can be dismissed without it.
They are best read in pairs, because each has a counterweight. Cui Bono asks who benefits, which is exactly the question Hanlon's razor tells you not to reach for first. Sattinger's Law, from the figures, warns that diagnosis skips the trivial cause, while the zebra rule warns against the exotic one. A reasoner who knows only one razor from each pair will be wrong in a predictable direction.
Rules for estimating and planning
A second set turns the evidence on bias into procedure. Reference Class Forecasting answers the planning fallacy by starting from how similar projects actually turned out. The Premortem asks a team to assume the plan has failed and explain why, which gets round the reluctance to criticise a plan in progress. Bayesian Updating, Cromwell's Rule, Fermi Estimation, Inversion and Second-Order Thinking are the rest of the kit: revise beliefs by how surprising the evidence is, never be certain, break an unknown into knowable parts, work backwards from failure, and ask what follows from the consequences.
Rules for disagreeing well
A third set is about arguing in a way that could change someone's mind, including your own. The Principle of Charity asks you to answer an argument in its strongest form. The Ideological Turing Test sets the bar at stating an opponent's view so well that its holders cannot tell you from one of them. Double Crux looks for the fact on which both sides would change their minds, and Keep Your Identity Small explains why some topics defeat all of these: people who have made a position part of who they are cannot concede it.
Change and the status quo
Two precepts in the group pull in opposite directions on a question that runs through politics. Chesterton's Fence says not to remove a fence until you know why it was put up, and Burke's prescription makes the same case for inherited practice. The Reversal Test, from Bostrom and Ord, checks the opposite error: if moving in either direction from the current position seems bad, the objection is probably attachment to the current position. Used together, they separate caution that respects accumulated knowledge from caution that merely defends what exists.
Where it applies
These are among the most portable ideas in the ledger. Doctors use the zebra rule and Osler's dictum every day. Forecasters and project boards use reference classes and premortems. Moderators, teachers and anyone running a hard conversation can use charity and the ideological Turing test. The Eisenhower Matrix and Skin in the Game apply to how organisations assign work and responsibility.
Where it is argued over
Every razor is a heuristic, and each can mislead. Hanlon's razor can excuse real malice, Hitchens's razor is often applied only to opponents' claims, and Occam's razor says nothing about which simplicity counts. The ethical precepts carry the oldest dispute of all: Kant's Categorical Imperative and Bentham's Felicific Calculus give different answers to the same cases, and Hume's Guillotine questions whether facts can settle such disputes at all. The ledger files each with its defenders' case and its critics'.
Why it is worth knowing
The evidence group shows how thinking fails; this group is the set of tools people have built in response. Most of them fit on a card, and most of them fail when used alone. The skill they teach is choosing the right rule for the question, and checking it against the rule that pulls the other way.
The entries behind this essay: Precept in Decision-making & Reasoning. Every subject has its essays on the essays page.