Chapter 5 focuses on the history of the philosophy of logic. Marcuse distinguishes three important moments in this history: 1) the original dialectic (the Socratic logic of Plato’s dialogues); 2) formal logic (Aristotle’s logic in The Organon); and 3) modern historical dialectical logic (from Hegel into Marxism).

The Platonic dialogues do not immediately appear to present formal logic. These are dialogues in which people converse; they do not present themselves as structured or formal and, instead, appear fluid and organic—they are, in essence, conversations. There is no explicit logic in the dialogues, and there is also no distinction between practical and theoretical. The dialogues begin with an experience and the assumptions that surround that experience. The dialogues move into dialectical thinking with the contradictions and tensions that emerge out of a reconsideration of these assumptions: We start at one point and see that it is not sufficient, and then continue to the next point, which is a revision of the first point by way of a consideration of the first point’s tensions. The goal of the dialectic is the search for truth, which proceeds by way of these tensions and subsequent revisions. Thus, tension and contradiction are the engines of truth seeking.