 it should now be obvious that one could in fact use proper parthood as an alternative starting point for the development of classical mereology, using the right-hand side of (24) as a definiens for ‘P’. This is, for instance, the option followed in Simons (1987), as also in Leśniewski’s original theory (1916), where the partial ordering axioms for ‘P’ are replaced by the strict ordering axioms for  ‘PP’.[9]  Ditto for ‘PE’, which was in fact the primitive relation in Whitehead's (1919) semi-formal treatment of the mereology of events (and which is just the converse of ‘PP’). Other options are in principle possible, too. For example, Goodman (1951) used ‘O’ as a primitive and Leonard and Goodman (1940) used its  opposite:[10]