Because trading has a random element, a good decision can lose and a bad decision can win on any given trade. Confusing the two — treating every loss as a mistake and every win as validation — teaches you exactly the wrong lessons. Separating outcome from decision quality is the mental shift that lets you actually learn.
The four outcomes
Every trade lands in one of four boxes: good decision + win (great, repeat it), good decision + loss (fine — correct process, unlucky outcome, repeat it), bad decision + loss (learn from it), and the dangerous one, bad decision + win (a reckless trade that happened to pay — the market just rewarded behavior that will eventually ruin you). Only two of the four are about your skill; the other two are variance.
Why the distinction matters
If you judge purely by P&L, you'll abandon a sound strategy after a run of good-decision losses (variance), and you'll reinforce reckless habits every time a bad-decision trade wins. The market is a noisy teacher: it doesn't reliably reward good decisions in the short run, so if you let it grade you trade-by-trade, it teaches you nonsense. You have to grade the decision yourself.
The market pays good and bad decisions almost at random in the short run. If you let your P&L pick your lessons, you'll learn the market's noise, not your craft.
Grading the decision
After each trade, ask not “did it win?” but “was it the right decision given what I knew?” A good-process loss earns a mental checkmark; a bad-process win earns a warning. This is the foundation of grading process over P&L, and it's what keeps you sane through the inevitable losing streaks and humble through the lucky wins.