 Robert Richardson (1979) argues that Fodor’s multiple realizability challenge to reductionism (see  §1.2  above) rests on a misunderstanding of Ernest Nagel’s “classic” account of intertheoretic reduction. Although Nagel’s (1961) detailed examples of historical cases of intertheoretic reduction in science all involve biconditional cross-theory “conditions of connectivity” (what Fodor and others called “bridge laws”), one-way conditional connections expressing sufficient conditions at the reducing level are all that his “principle of derivability” requires. Richardson cites passages from Nagel (1961) indicating that Nagel himself saw the force of this point. Multiple realizability only challenges necessity (and nondisjunctive) reducing conditions, and so is not a challenge to a projected, distinctively Nagelian reduction of psychology to some physical science. However, this response seems not to dispose of the original argument’s force against mind-brain identity theory.