 Most importantly, the Demonstration makes great use of the principle of sufficient reason, which motivates the cosmological argument, and he explicitly and repeatedly avows it in the correspondence with Leibniz (C 3.2, W 4.606). It is not mentioned in the correspondence with Collins, but he there adopts principles that can be derived from it. Clarke’s understanding of the principle of sufficient reason differs notably from Leibniz’s formulation, with whom it is more frequently associated. This was a major source of contention in their correspondence. Clarke asserts that the sufficient reason why something exists as it does may be due to the “mere Will” of God and nothing more (C 3.2, W 4.606–607; C 5.124–130, W 4.700). This involves two claims. First, in cases of complete indifference (such as God choosing where to place the world in the infinite expanse of absolute space), God is capable of acting even if there is no reason to prefer one option over another. Second, a free will is able to refrain from acting on what reason presents to it as best to do. As a consequence of these, Clarke denies the identity of indiscernibles. This is significant for Clarke’s Newtonianism, because if space is real and absolute, then the identity of indiscernibles must be false because regions of space are indiscernible with respect to their intrinsic and (prior to the creation of the world) their extrinsic properties. Clarke may also have felt the need to accommodate indiscernible atoms, which Newton seemed to allow. (Clarke defends atomism in the letters to Leibniz, but in his other works he claims that all matter is infinitely divisible.) Because Clarke denies the identity of indiscernibles and affirms libertarianism, Leibniz claims that Clarke grants the principle of sufficient reason “only in Words, and in reality denies it. Which shews that he does not fully perceive the Strength of it” (L 3.2, W 4.601). In response, Clarke argues that if Leibniz is right then a free agent would be merely passive because determined to do what reason presents, but a “passive agent” is a contradiction since the concept of agency includes the concept of activity. Leibniz was never satisfied with Clarke’s position and by the fifth letter he was more explicit than previously that the principle of sufficient reason and the principle of the identity of indiscernibles are not independent but the latter is derived from the former (L 5.21), and in the fifth letter’s cover letter, sent to Princess Caroline, Leibniz asserts, “If [Clarke] does not entirely accept the received great principle that nothing happens without there being a sufficient reason why it happens thus rather than otherwise, I could not help doubting his sincerity, and if he grants it then farewell to the philosophy of Monsieur Newton” (quoted in Khamara 2006, 4).