 The idea is that if what it is to be each A property entails that the A-properties are uniquely realized by the B-properties, this amounts to a kind of reducibility of the A-properties. Consider an example: one might take oneself to have offered a reduction of the number one, in claiming that: what it is to be the number one is just to be the successor of zero. One important contrast with the identity conception is that on the essentialist conception, successful reductions reveal metaphysical structure. Thus, one might say in our example that the number one is ‘built out of’ the number zero and the successor function.