Spinoza's image of thought: ratio and the example of the fourth proportional

What constitutes the image of thought for Spinoza? The “image of thought” is a term Gilles Deleuze uses to describe how a philosopher represents thinking, implicitly and pre-theoretically. It refers to what a philosopher presupposes about thinking. Spinoza presupposes that human beings think, and that thinking comes in three kinds: imagination, reasoning, and intuition. This suggests that thinking may be represented as a ladder ascending from sensory perception to a Godlike form of intellectual intuition. Yet the image Spinoza actually uses is the geometrical example of finding the fourth proportional, which does not illustrate anything of the kind. In this paper I will explore what this example reveals about Spinoza’s epistemic presuppositions, and suggest that his image of thought is one of finding the right ratio.