 Critics of multiple realizability and the standard arguments it has figured into also extended their reach early in the twenty-first century. Lawrence Shapiro (2008) raises some methodological difficulties involved in testing whether a given psychological kind actually is multiply realized. (For a related argument see Thomas Polger 2009.) Shapiro reminds us of the crucial role that auxiliary assumptions play in hypothesis testing generally (within a broadly hypothetico-deductive model), and considers a collection of explicit auxiliary assumptions that might be implicitly used to establish a multiple realization hypothesis. He presents a recent ferret brain-rewiring experiment as a scientific example, in which axonal inputs from the primary visual tract were redirected in ferret embryos to project to primary auditory cortex (see Sharma, Angelucci, &amp; Sur 2000 for the scientific details). An auxiliary hypothesis requiring multiply realized higher-level (in this case, psychological) kinds to be “exactly similar”—identical—across distinct realizers won’t help the proponent of the standard multiple realizability argument with this purported case. It is easy to measure better visual performance in the normally-wired control ferrets compared to the re-wired experimental animals. While the experimentally re-wired animals have some visual function, it is diminished significantly compared to controls. “Exact similarity” (identity) of visual function is thus not present across these groups. On the other hand, one might argue for the multiple realizability premise in this ferret re-wiring case using an auxiliary hypothesis that only requires similarity in multiply realized higher level properties, yet still requires that differences across the realizers should not be limited only to the differences that cause differences in the realized (in this case, visual) properties. Shapiro speculates that this auxiliary assumption seems best to capture the sense of multiple realization stressed by proponents of the standard argument. But if we adopt it, again the ferret re-wiring case seems not to provide an empirical instance of multiple realization. Shapiro remarks: “the differences in ferret brains explain nothing more than differences in ferret visual properties” (2008: 523). Shapiro also argues that his detailed discussion of hypothesis testing difficulties for any multiple realization hypothesis reveals a flaw in Bechtel and Mundale’s (1999) influential criticism (discussed in  §2.3  above). Bechtel and Mundale’s examples, drawn from the comparative functional neuroanatomy of vision, only compared homologous brain structures. But these only have differences that make a difference in their visual properties, nothing else. Instead of these examples, Shapiro insists, “one should be looking at different brains that reveal similar visual properties despite their differences” (2008: 524)—exactly the kinds of evidence that Bechtel and Mundale’s emphasis on homologies doesn’t consider.