The principle of Summation, which is a technically sharpened version of the familiar claim that a whole is a sum of its parts, is presented by Peter van Inwagen as a trivial truth. I argue to the contrary, that it is incompatible with the natural assumption that a whole may gain or lose parts non-instantaneously. For, as I show, the latter assumption implies that something can be determinately a whole without being determinately a sum of parts, and this, in turn, indicates the falsity of Summation. I point out that the tension between Summation and the possibility of non-instantaneous gain or loss of parts compels us to rethink the relations between the concepts of whole and sum, and may have far reaching consequences for the mereology of physical objects.
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