This paper aims at explicating aleatory materialism proposed by Althusser in the 1980s. The first part starts with providing a concise account of the meanings and functions associated with the concept ＂void＂ in Althusser's writings before the mid-1970s. This will be followed by an outline of the main features that characterize aleatory materialism. The second part illustrates the ways in which aleatory materialism respond to the tensions inherent in Althusser's earlier writings and how it can be understood as a new practice of philosophy, or better still, a ＂non-philosophy＂. The final part concentrates on Althusser's reading of Machiavelli, and contends that Althusser eventually failed to formulate a political imaginary compatible with the radicalism of his aleatory materialism.