What, for Spinoza, would constitute an equal society? I would like to address this question not on the basis of what Spinoza appears to say about equality in his political texts, nor on the basis of the egalitarianism that others have ascribed to him, but on the basis of his own concept of equality. In this chapter I will develop the view that, for Spinoza, the concept of equality derives from Euclid, and is deeply connected to the complex concept of ratio.

As the chapters in this volume make clear, ratio is central to Spinoza's philosophy. In Latin, ratio has at least three meanings. First, and most obviously, ratio means reason: reason is the second kind of knowledge, through which we understand things adequately and truly. Ratio can also mean the reason why something exists or acts as it does: its causal or explanatory story. Every individual has a ratio that fully explains it, the full reason or rationale for that thing being what it is. Our own reasoning involves understanding the causes of things, or the reasons why they are as they are. When we understand a thing through ratio, we understand the ratio of that thing.

Secondly, ratio can mean relation. We are constantly in relation to the other people and things that constitute our world and that we encounter in experience. Indeed, one of Spinoza's first claims about finite modes is that they necessarily relate to one another (E IP28). No finite, durational thing can exist or cause effects unless it is determined to exist and cause effects by another finite, durational thing, and so on, to infinity. A finite body and a finite mind exist, act, and think by virtue of their relations with other finite bodies and minds. The more relations a body enters into, the more ideas its mind – which enters into relations with the ideas of those things – is capable of perceiving. The highly composite and affective human body has a high degree of relatability, both internally, in terms of the interrelation of its constituent parts, and externally, in terms of its relations to other things. As our bodies are constantly exchanging affects with other bodies, and our minds are constantly exchanging ideas with other minds, we are constituted by these relations.