In The Poverty of Historicism, Popper claimed that because the growth of human knowledge cannot be predicted, the future course of human history is not foreseeable. For this reason, historicist theories like Marxism are unscientific or untrue. The aims of this article are: first, to reconstruct Poppers argument, second, to defend it against some critics, and third, to show that it is itself based a weak form of historicism.