The analogy!
To Heinrich, the ineffable sensations a fit runner experiences on a good 10-miler are similar to the joy he thinks migratory birds must have when flying. He writes, “What makes the blackpoll warbler strike out south in the fall after a cold front is probably not fundamentally different from what motivates me to jog down a country road on a warm, sunny day. We both respond to ancient urges that had adaptive roots.”
Contested Realities 