Free public beta until December 4, 2026: every course is open, no card needed. After that, the first chapter of each course stays free and full access is a one-time $39 ($19 for up to 100 founding members chosen for helpful feedback). Your progress is kept. How the beta works

counterexample

A single object disproving a universal claim.

A counterexample to $\forall x,P(x)$ is one object $c$ in the domain with $P(c)$ false.