1. Leibnizian substance: Something is a substance if and only if it evolves by the fundamental laws
2. Russellian laws: The cosmos is the one and only thing that evolves by the fundamental laws
3. Spinozan monism: The cosmos is the one and only substance (from 1 and 2)
As Schaffer is well aware, there is lots of irony in all of this. (At NewAPPS we have discussed Russell's reservations about Spinoza several times here, here, and also Jeff. [Recall also Russell's debts to Boole on Clarke vs Spinoza; and Stebbing on Spinoza.]) Now, my objection to this argument is inspired by my reading of Spinoza's so-called "Letter on the Infinite," but what follows is not meant to be a historical argument (or a gotcha, 'you got the history wrong' moment). Recall that I read Spinoza as claming that characterizing and grasping substance as such does not involve our ordinary scientific 'utensils' (e.g., measures, mathematics, laws of nature), but rather concepts like essence and eternity. Mathematical physics can only give a partial view of substance as such. Now one reason for this is that mathematical physics of Spinoza's day, treats some part of nature as a closed system (governed by its own 'conservation' rules/laws). Moreover, Spinoza would deny that fundamentally the universe evolves. For, applying temporal concepts to the universe is, however useful it may be, always a less than fully adequate conceptualization of the universe.