Epidemiology: Everything Turns on One Number Crossing One
If each case produces more than one further case, an outbreak grows; below one, it fades. That threshold is simple — and almost everything else about the subject is context-dependent.
Concept
Compound Interest
Amara's Law: Too Much in Two Years, Too Little in Twenty
The same technology gets overestimated early and underestimated late. Both errors come from drawing a straight line through something that is slow first and rapid afterwards.
Neuroscience: The Brain Rearranges Itself Around What You Do
The computer analogy breaks down at the most important point — the hardware physically changes with use. That single property explains most of what is practically known about learning.
Longevity and Health: The Big Effects Are the Ones Nobody Sells
The interventions with the largest effects have been known for decades and are unglamorous. Attention flows almost inversely to effect size, for a simple reason — the big ones cannot be sold.
Personal Finance: The Boring Part Is the Part That Works
Most attention goes to choosing investments — the part you control least and that matters least. Saving rate, time, and costs decide the outcome, and all three are dull.
Half-Life: Why the Last Bit Takes So Long to Go
An afternoon coffee still has an eighth of its caffeine in you the next morning. Anything that loses a constant proportion per period behaves this way — including skills and knowledge.
Opportunity Cost: The Free Ticket That Costs You an Evening
Money spent arrives with a bill. The alternative you silently rejected never does — which is why we count one half of every decision carefully and the other half not at all.
Compound Interest: The Pond Was Empty Until It Wasn't
A lily pad covering a pond on day thirty covered half of it on day twenty-nine. Nothing about its growth changed — only the base it grew from. Human intuition fails at this reliably and expensively.
Technical Debt: The Loan Nobody Remembers Taking
Cunningham's metaphor was never about bad code. It was about a rational trade — ship now, pay interest later — and the interest is levied on every future change you make.
Markets: A Price Is a Bet, Not a Measurement
A company posts record profits and the share price falls. That stops looking irrational once you see that a price already contains what everyone expected — and moves only on the difference.