 In explaining the thesis that there is no inter-substantial causation between finite substances in the Monadologie, Leibniz famously says: “monads have no windows through which something can enter or leave” (G 6:607; AG 214). This metaphor of being “windowless” helps the reader attain some understanding of Leibniz’s thesis, but it will be an imperfect understanding to the extent that it involves conceiving of monads as being extended.