 In opposition to the empiricist model of knowledge production, Althusser proposes that true or scientific knowledge is distinguished from ideology or opinion not by dint of an historical subject having abstracted the essence of an object from its appearances. Instead, this knowledge is understood to be produced by a process internal to scientific knowledge itself. Though this transformation takes place entirely in thought, Althusser does not maintain that scientific knowledge makes no use of facts. However, these facts or materials are never brute. Rather, specific sciences start with pre-existing concepts or genera such as “humors,” “unemployment,” “quasars,” or “irrational numbers.” These genera may be ideological in part or in whole. Science’s job is to render these concepts scientific. This labor is what Althusser terms “theoretical practice.” The result of this practice is scientific knowledge. Scientific knowledge is produced by means of applying to these genera the body of concepts or “theory” that the science possesses for understanding them. This body of concepts may be more or less unified and consistent and it may be more or less consciously articulated. Further, the sum of the individual concepts that this theory consists of delimits the possible ways in which the genera that a science begins with can be understood.