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Ideology is not always taken seriously as an existing practice: but to recognize this is the indispensable prior condition for any theory of ideology.  The existence of a theoretical practice is taken seriously even more rarely: but this prior condition is indispensable to an understanding of what theory itself, and its relation to ‘social practice’ are for Marxism.

    …By theory, in this respect, I shall mean a specific form of practice, itself belonging to the complex unity of the ‘social practice’ of a determinate human society.  Theoretical practice falls within the general definition of practice.  It works on a raw material (representations, concepts, facts) which it is given by other practices, whether ’empirical’, ‘technical’ or ‘ideological.’

Louis Althusser, “On the Materialist Dialectic,” For Marx (1965)