During the Cold War and especially after the failure of Stalinism, Althusser tried to give a contribution to build up a new Marxism by the production of an original "theory of ideology". The result was the draft of a new political philosophy: the so-called "aleatory materialism" that is a philosophy of contingency (or a philosophy of the un-necessary). Using at the same time Machiavelli, Spinoza and Marx, Althusser proposed an explanation of the birth of society and State with reference neither (...) to the doctrine of natural law nor to contractualism. In the althusserian theory, the ideology (persuasion without constraint) is necessary in order to interpellate the individuals into subjects. But the ideology can fail because the class struggle is aleatory in its effects. So, beyond every kind of functionalism, a revolution is always possible starting from a deviation (clinamen), that may be the prelude of a structural yielding and so an occasion to transform the old structures or to construct something totally new. (shrink)