Nothing is more commonplace these days than discussions of the relation between theory and practice — or praxis as it has become fashionable in some circles to spell it¹. Less commonplace is the acknowledgment that theory is itself a practice, one among a number of social practices which combine and interweave in most complicated ways in any social structure. It is to the work of the group of French Marxists led by Louis Althusser that we owe the use of the phrase "theoretical practice', as the symbol of this approach to the understanding of the relations between theory and the other practices. Althusser has devoted much attention to that particular theoretical practice he denotes by "scientific practice'; and such other workers in the same general tradition as Badiou, Lecourt, Raymond, Fichant and Pécheux, to name a few, have tried to develop and refine (even reverse!) some of Althusser's insights.* This group thus represents an active current within contemporary Marxist thought, and the group is especially interested in problems of the epistemology of the sciences. Without wishing to tie myself too closely to any one of the varying shades of interception of this group, nor burden them with my particular nuances, I should like to discuss some themes of this approach to the sciences, and give an example of how it may help a working (or should I say practicing?) theoretical physicist to try and avoid some of the pitfalls inherent in current ideologically inspired misinterpretations of certain problems in theoretical physics.