If I had to build a whole book around one principle, it would be this: you cannot think your way to an accurate view of yourself or the world from inside your own head — you have to build external feedback loops and then actually submit to them.

Most self-improvement books fail because they're closed systems. They give you a framework, you apply it in your imagination, you feel insight, and nothing changes — because the feeling of insight is generated by the same mind that produced the problem. There's no error signal coming in from outside. The person journaling about their flaws is grading their own homework.
So the book would argue that the unit of change isn't willpower or mindset — it's the loop. Ship the essay and read the criticism. Make the prediction and score it later. Tell someone your plan and let them hold you to it. Weigh the thing, time the thing, count the thing. The discomfort of external feedback isn't a side effect of growth; it's the mechanism. Everything that feels like growth but doesn't route through some channel that can tell you no is probably rehearsal, not change.
The corollary — and this is the part I think most books dodge — is that you'd have to design your life so that being wrong is cheap and frequent rather than rare and catastrophic. Small bets, fast loops, low ego-stakes per iteration. People avoid feedback because each instance feels like a verdict on their worth; the fix is structural, not motivational.