 This condition is, under a certain interpretation, what underlies Nagelian bridge-principles (van Riel 2011, see however Sarkar 2015 for a different interpretation). It is also explicitly endorsed by Robert Causey (1972a, b, 1977), who connects it to theory reduction via micro-reductions. Just as in the writings of Smart (1959, 1963), the idea is that the relevant identities are not a priori. In addition, he suggests that by identifying the relevant kinds, attributes and properties of the reduced theory with kinds, attributes and properties of the reducing theory, we arrive at explanatory reduction because the laws of the respective theories will thereby be shown to be nomologically equivalent (Causey 1974: 5).