“Althusser’s Lenin” is an examination of the importance of Lenin in the development of Althusser’s thought. His initial definition of philosophy as the theory of theoretical practices necessarily set beyond the realm of political practice was in some sense defined in opposition to Lenin whose conception of philosophy Althusser regarded as failing to achieve the necessary degree abstraction. But a close reading of Lenin’s conception of the historical contradiction that produced the Russian Revolution led Althusser to see in Lenin a thinker of immanent causality and the philosophy of the encounter. More than anything else, however, he finally came to see through Lenin that the power of philosophy lay in the simple act of drawing a line of demarcation, the philosophical equivalent of splitting the atom, to transform the landscape of history.