 Challenges to claimed multiple realizations can attack either step, and most importantly, the step challenged can differ from case to case. Successfully challenging either step blocks any multiple realizability argument appealing to that kind. As we saw in the previous section, Bechtel and Mundale (1999) describe cases in which cognitive neuroscientists treat the physical realizers (brain states) as type-identical across species, thus attacking step (i). In other cases—Couch’s example is primate versus octopus eyes—one can appeal to easily-found differences in functional properties, attacking step (ii). The two types of eyes have different visual pigments in their photoreceptors, different retinas, and different ways of focusing light. These physical differences lead to straightforward input-output (functional) differences: in the optic stimuli the two eyes respond to, in reaction times, and more. Their functions may be similar, but similarity isn’t identity and the multiple realization argument since Putnam’s original version (see  §1.2  above) requires identity at the realized functional kinds. Cross-species functional similarities are often quite superficial, especially across species from widely differing taxa (a point stressed by both Couch and Shapiro). In actual scientific practice, discovered physical (neural) differences typically incline psychologists to search for functional differences. Couch’s point is that the individuation of psychological states, like the individuation of brain states, is an empirical issue; Shapiro and Couch both suggest that claims about multiple realizability often rely heavily on “folk” psychological intuitions about individuating mental kinds.