"Blood-purity doctrine is stupid, because it ignores facts. In Daphne’s and my year, the smartest student is a Muggle-born. Purebloods’ theories say this can’t happen. I’ve Merlin-level magical power, and I’m a half-blood. Blood-purity doctrine says this can’t happen either. Both Vincent Crabbe and Gregory Goyle are stupid and they’re magically weak, but this supposedly can’t happen because they’re Purebloods. And only Pureblood couples give birth to Squib children; if either parent is a half-blood or a Muggle-born, Squib children don’t happen."