Mereology without Weak Supplementation

According to the Weak Supplementation Principle (WSP)--a widely received principle of mereology--an object with a proper part, p, has another distinct proper part that doesn’t overlap p. 1 A guiding thought behind WSP is that there must be some mereological difference between a composite and any one of its proper parts and there would be no such difference if something were composed by a single proper part [Simons 1987: 26 fn. 17; Casati and Varzi 1999: 38]. I think that WSP is false. Indeed, in my view, the main argument of Nikk Effingham’s and Jon Robson’s recent article in this journal, ‘A Mereological Challenge to Endurantism’ [2007: 633-640], should be taken to show that WSP is false. Effingham and Robson, hereafter ‘E&R’, see matters differently; they take themselves to have provided a compelling objection to endurantism. My reply to E&R’s objection serves not only to defend endurantism but also bears on mereology in general. First, I argue that denying WSP can be motivated apart from the truth of endurantism. I then go on to offer an explanation of WSP’s initial appeal, argue that denying WSP fails to have untoward consequences for the rest of mereology, and show that the falsity of WSP turns out to be consistent with the above cited thought behind it.