 The first realization definition has been explored and defended in most detail by Andrew Melnyk (see Melnyk 2003 and the references therein). For Melnyk, a property F realizes a property G if and only if (a) G is identical to a second-order property, the property of having some property that has a certain causal or theoretical role; and (b) F is the property that plays the causal or theoretical role in question. We may call this notion ‘second-order realization’ to distinguish it from a different notion of realization to be considered in a moment. This suggests: 