What the Frog's Eye Tells the Frog's Brain

One of the early classics of cognitive science is the famous paper "What the Frog's Eye Tells the Frog's Brain," by J. Y. Lettvin and colleagues (1959). It showed that the frog's visual system is sensitive to small moving dark spots on the retina, tiny shadows cast in almost all natural circumstances by flies flying in the vicinity. This "fly-detector" mechanism is appropriately wired to the hair trigger in the frog's tongue, which handily explains how frogs feed themselves in a cruel world and thereby help propagate their kind. Now what does the frog's eye tell the frog's brain? That there is a fly out there, or that there is a fly-or-a-"slug" (a fake fly of one sort or another), or a thing of kind K (whatever kind of thing reliably triggers this visual gadget)? Darwinian meaning theorists (such as Ruth Millikan, David Israel, and I) have discussed this very case, and the arch-critic of evolutionary theory, Jerry Fodor, has pounced on it to show what is wrong, by his lights, with any evolutionary account of such meanings: They are too indeterminate. They fail to distinguish, as they ought, between such frog-eye reports as "fly here now" and "fly or small dark projectile here now" and so forth. But this is false. We can use the frog's environment of selection (to the extent that we can determine what it has been) to distinguish between the various candidates. To do this, we ask which conditions in the frog’s environment shaped the mechanism and sustained its role in feeding. And to the extent that there is nothing in the selective environment that uniquely singles out a particular class of occasions, there is also no fact of the matter about what the frog's-eye-report really means. This can be brought home vividly by sending the frog to Panama—or more precisely, sending the frog to a novel selective environment.

Suppose scientists gather up a small population of frogs from some fly-grabbing species on the brink of extinction, and put them under protective custody in a new environment—a special frog zoo in which there are no flies, but rather zookeepers who periodically arrange to launch little food pellets past the frogs in their care. To the keepers' delight, the system works; the frogs thrive by zapping their tongues for these pellets, and after a while there is a crowd of descendant frogs who have never seen a fly, only pellets. What do their eyes tell their brains? If you insist on saying the meaning hasn't changed, you are in a bind. The case of the frogs is simply an artificially clear instance of what happens in natural selection all the time: exaptation— the reuse of an existing structure for a new function. As Darwin was careful to remind us, the reuse of machinery for new purposes is one of the secrets of Mother Nature's success. We can drive home the point, to anyone who wishes some further persuasion, by supposing that not all the captive frogs do equally well. Variation in pellet-detecting prowess in their visual systems causes some to eat less heartily than others, and then to leave less progeny as a result. In short order there will have been undeniable selection for pellet detection—though it would be a mistake to ask exactly when enough of this has occurred for it to "count." Don't listen for a bell to ring indicating that what the frog's eye is saying has just now been revised. There was no Prime Mammal and there is no Prime Pellet Detection.

Unless there were "meaningless" or "indeterminate" variation in the triggering conditions of the various frogs' eyes, there could be no raw material (blind variation) for selection for a new purpose to act upon. The indeterminacy that Fodor (and others) see as a flaw in Darwinian accounts of the evolution of meaning is actually a precondition for any such evolution. The idea that there must be something determinate that the frog's eye really means—some possibly unknowable proposition in froggish that expresses exactly what the frog's eye is telling the frog's brain—is just essentialism applied to meaning (or function). Meaning, like function, on which it so directly depends, is not something determinate at its birth. It arises not by saltation—huge leaps in Design Space—or special creation, but by a (typically gradual) shift of circumstances.