 Such a conception of the nature of a sufficient reason have lead some commentators to think that for Leibniz the PSR is a logical notion or that it is a metaphysical notion that is ultimately reducible to logic (Couturat 1901: 123ff and Russell 1937: v). But the notion of a sufficient reason as a non-terminating proof sequence is not the only conception of a sufficient reason to be found in Leibniz. And other conceptions have a decidedly less logical and more metaphysical flavor.