Orthodoxy, like heterodoxy, lies largely in its beholders’ eyes. Across the nineteenth century, Marxian economics contested the orthodoxy of classical political economy much as socialism contested capitalism. Across the twentieth, Marxian economics struggled to redefine its specific heterodox difference as orthodoxy swung between neoclassical and Keynesian economics, capitalism alternated between regimes of less and more state economic intervention, and classical socialism peaked. Its new reformulation articulates a self-consciously non-determinist theory of class (defined in terms of the production, appropriation and distribution of surpluses).  Such a Marxian economics represents a new, systematic, and well-developed alternative to both neoclassical and Keynesian economics.