In this article, I discuss how Leibniz’s first correspondence with Malebranche from early 1676 can shed new light on the notorious “all-things-are-one”-passage (ATOP) found in the Quod ens perfectissimum sit possibile from late 1676—a passage that has been taken as an expression of monism or Spinozism in the young Leibniz. The correspondence with Malebranche provides a deeper understanding of Leibniz’s use of the notions of “real distinction” and “separability” in the ATOP. This forms the background for a discussion of Leibniz’s commitment to the monist position expounded in the ATOP. Thus, on the basis of a close analysis of Leibniz’s use of these key terms in the Malebranche correspondence, I provide two possible, and contrary, interpretations of the ATOP, namely, a “non-commitment account” and a “commitment account.” Finally, I explain why I consider the commitment account to be the more compelling of the two.