The evolutionary liberalists,such as Hayek,think that there lie two kinds of rationalism. One is evolutionary rationalism,which bases upon correct presupposition. The other is constructivist rationalism,which bases upon false presupposition. In Hayek's eyes Marxism belongs to the latter. In this thesis we criticize such dichotomy raised by Hayek. Hayek's criticism on constructivist rationalism is not applicable for Marx's theory. In fact Marx upholds neither constructivist rationalism,nor evolutionary rationalism. His position should be called practical rationalism,which surpasses the former two positions. On practical rationalism Marx finds stable foundations for human reason.