Think clearer. Decide better.
Mental models, cognitive biases and decision tools, every one explained the same way: a plain definition, where it comes from, how to use it step by step, real examples and the mistakes to avoid.
Five sections, one structure
Every page has the same parts in the same order, so you can scan it in a minute or read it in ten.
The ideas people look up most
Cognitive Dissonance
Cognitive dissonance is the mental discomfort you feel when two of your beliefs, or a belief and an action, contradict each other. Leon Festinger described it in 1957. The discomfort pushes you to restore consistency, and the easiest route is often to change the belief or explain the evidence away rather than change the behavior.
Cognitive biasConfirmation Bias
Confirmation bias is the tendency to search for, interpret and remember information in a way that supports what you already believe, while overlooking or discounting evidence against it. It affects how you ask questions, which sources you trust and what you recall later, and it works mostly without you noticing.
Mental modelFirst Principles Thinking
First principles thinking is a way of reasoning that breaks a problem down to its most basic facts, the things you know are true without relying on convention or analogy, and then rebuilds a solution from those facts. It replaces "how is this usually done?" with "what is actually required?"
Mental modelSecond-Order Thinking
Second-order thinking is the habit of looking past the immediate result of a decision to the effects that follow from it, and the effects that follow from those. Where first-order thinking asks "what happens next?", second-order thinking keeps asking "and then what?" until the picture includes how people and systems react.
Mental modelInversion
Inversion is a mental model that approaches a problem from the opposite end: instead of asking how to succeed, you ask what would guarantee failure, then avoid those things. It also means testing a belief by asking what would have to be true for the opposite to be right. Charlie Munger made it famous in business.
Cognitive biasHeuristics
Heuristics are mental shortcuts: simple rules of thumb that let you make judgments and decisions quickly with limited information, time and attention. They usually work well enough, which is why the brain relies on them, but in some situations they produce predictable errors known as cognitive biases.
Four frameworks that tie it together
Definitions are everywhere. What's missing is a way to use them on a real decision. These four run through every page, tool and product here.
The Mentarium Decision Loop
Frame, Map, Stress-test, Decide, Review. The process behind every page, tool and product here.
Decision Weight Score
Reversibility × impact × uncertainty, scored 1 to 10. It tells you how much process a decision deserves, so small choices stay fast.
OpenBias Radar
The 7 biases that most often distort career, money, relationship and business decisions, with the question that catches each.
OpenModel Stack
Three models used together for one situation. Quitting a job: opportunity cost + regret minimization + reversible decisions.
Open30 questions. 20 minutes. Your result, free.
Number and letter series, analogies, logic and patterns. Scored instantly with every answer explained. No sign-up, no email, no paywall on the result.
Tools for the decisions that matter
The Mentarium Field Guide
60 Mental Models for Clearer Thinking and Better Decisions
$14$19
- 60 mental models, 1 to 2 pages each: in one line, when to use it, trigger questions, an example, the trap
- The 25 biases that break good decisions, each with the model that fixes it
- 12 Model Stacks for real situations: quitting a job, hiring, investing, moving cities and more
Mentarium Thinking Skills
12 skills that make Claude and ChatGPT think with mental models
$29
- 12 Claude skills (SKILL.md) for Claude.ai, the Claude apps and Claude Code
- The same 12 as ChatGPT Custom GPT / Project instructions, plus a knowledge file with the Mentarium frameworks
- Install guide for both, in plain steps
The Mentarium Decision Journal
Log decisions, review them, and see how well calibrated you are
$29
- An Obsidian vault: plain Markdown files you own, no account
- A decision template guided by the Decision Loop, with the Decision Weight Score
- Review Queue: decisions whose review date has come
Bring the library into your AI chat
The Mentarium Decision Coach is a free connector. Describe a decision and your assistant runs the Decision Loop with you, picks the Model Stack and checks your reasoning with the Bias Radar.