Russell’s first contact with mereology occurred probably in the time around 1899 during his investigations on the philosophy of Leibniz. In the early idealistic phase (1895-1899) he did not reflect much about the parthood relation, because it “has been wrapped in obscurity – though not without certain more or less valid logical reasons – by writers who may be roughly called Hegelian” (1903: §133). The main ‘logical reason’ for this is the idealistic doctrine Russell begins to reject at this time, namely, that analysis (decomposition of a whole in its parts) implies falsification, i.e. only a theory of the reality as a unified whole can be true (see Russell 1918: 178). But even recognising some importance of mereological analysis after 1900, he never accepted it to be so fundamental as set theory or predicate logic (1903: § 139).  …

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