After describing Wittgenstein's notion of "certainty", in this article I provide four arguments to demonstrate that no certainty can be acquired at will. Specifically, I argue that, in order to assimilate a certainty, it is irrelevant whether the individual concerned (1) has found a ground that seemingly justifies that certainty; (2) has a given mental state; (3) is willing to accept the certainty on the proposal of a persuader; or (4) tries to act according to the certainty involved. Lastly, I analyse how each of these arguments is reflected in the way children acquire certainties.