Philosopher Andy Clark gives a wonderful example of the blue-fin tuna's physicality. He says that the tuna has the incredible ability to navigate gracefully through water, to turn quickly, and to start swimming quickly from a standstill. However, the actual muscle abilities of the tuna are too weak by about a factor of seven to actually perform these tasks all by itself; instead the tuna is taking advantage of natural currents of the water. What point does he say this example illustrates?