ABSTRACT
Several previous studies 1 have examined the situation where young subjects are presented with two equal collections, A and B, and then n elements are transferred from A to B. What has been shown is that after the transfer young children expect the difference between the two collections to be of magnitude n rather than 2n. It is as if they forget that the n elements added to B are, in fact, the same elements taken away from A. However elementary, this commutability only tardily imposes itself with necessity on the subject.
