Spinoza asks: What can a body do? We call the latitude of a body the affects of which it is capable at a given degree of power, or rather within the limits of that degree. Latitude is made up of intensive parts falling under a capacity, and longitude of extensive parts falling under a relation. [TP: 256-7]
We know nothing about a body until we know what it can do, in other words, what its affects are, how they can or cannot enter into composition with other affects, with the affects of another body, either to destroy that body or to be destroyed by it, either to exchange actions and passions with it or to join with it in composing a more powerful body. [TP: 257]