  That substance should be called upon to account for both difference and sameness in the world indicates an inherent tension in the concept.  Certainly, the two senses of the term ‘substance’ were in tension during the seventeenth century. The momentum of rationalist argument was to resolve the tension by folding the first sense into the second: there is no real differentiation in the world, only the appearance of difference. Seventeenth century rationalists assigned to substance three roles of connection. Substance was taken (1) to connect attributes as attributes of the same thing at a time (a given shape and a given size as the shape and size of the same thing), (2) to connect them over time (the later shape and size, perhaps different from the earlier, as nonetheless the shape and size of the very same thing), and (3) to connect them as somehow related to the thing as a certain kind of thing (for the Cartesians, shape and size would indicate the thing to be of the kind “extended”). However, Spinoza alone among the continental rationalists fully embraced the conception of substance as a fundamental connection between things. The other members of the movement struggled to retain a notion of substance as individuator, but did so with varying degrees of success.