Deleuze writes of Spinoza’s epochal realization that we do not know what a body can do:
Spinoza will engender all the passions, in their details, on the basis of these two fundamental affects: joy as an increase in the power of acting, sadness as a diminution or destruction of the power of acting. This comes down to saying that each thing, body or soul, is defined by a certain characteristic, complex relation, but I would also say that each thing, body or soul, is defined by a certain power [pouvoir] of being affected. Everything happens as if each one of us had a certain power of being affected. If you consider beasts, Spinoza will be firm in telling us that what counts among animals is not at all the genera or species; genera and species are absolutely confused notions, abstract ideas. What counts is the question, of what is a body capable? And thereby he sets out one of the most fundamental questions in his whole philosophy (before him there had been Hobbes and others) by saying that the only question is that we don’t even know [savons] what a body is capable of, we prattle on about the soul and the mind and we don’t know what a body can do. But a body must be defined by the ensemble of relations which compose it, or, what amounts to exactly the same thing, by its power of being affected. As long as you don’t know what power a body has to be affected, as long as you learn like that, in chance encounters, you will not have the wise life, you will not have wisdom.
Knowing what you are capable of. This is not at all a moral question, but above all a physical question, as a question to the body and to the soul. A body has something fundamentally hidden: we could speak of the human species, the human genera, but this won’t tell us what is capable of affecting our body, what is capable of destroying it. The only question is the power of being affected. What distinguishes a frog from an ape? It’s not the specific or generic characteristics, Spinoza says, rather it’s the fact that they are not capable of the same affections. Thus it will be necessary to make, for each animal, veritable charts of affects, the affects of which a beast is capable. And likewise for men: the affects of which man is capable. We should notice at this moment that, depending on the culture, depending on the society, men are not all capable of the same affects.