Pay yourself first — save at least one-tenth of everything you earn

The book's foundational law, delivered through the story of Arkad the once-poor scribe, is deceptively simple: before paying anyone else, set aside at least a tenth of your income for yourself, treating your own future wealth as the first and most important bill you owe. Arkad discovers this principle from a wealthy moneylender, who dismisses the idea that wealth requires special luck or intelligence — it requires this one habit, consistently applied.

The parable insists this isn't a matter of having enough left over after expenses; it's a matter of deciding the tenth is untouchable before expenses get the chance to expand and consume it, since Clason observes that spending has a natural tendency to rise to meet whatever income is available unless something is deliberately walled off first.

Everything else in the book builds on this single habit — none of the later advice about investing matters if there's never any surplus to invest. Takeaway: decide your savings rate before you decide your spending.