The indeterminancy of fungal growth is one of the most exciting things about fungi. Human bodies achieve a determinate form early in our lives. Barring injury, we’ll never be all that different in shape than we were as adolescents. We can’t grow extra limbs, and we’re stuck with the one brain we’ve each got. In contrast, fungi keep growing and changing form all their lives. Fungi are famous for changing shape in relation to their encounters and environments. Many are “potentially immortal”, meaning they die from disease, injury or lack of resources, but not from old age. Even this little fact can alert us to how much our thoughts about knowledge and existence just assume deteriminate life form and old age. We rarely imagine life without such limits – and when we do we stray into magic. Rayner challenges us to think with mushrooms, otherwise. Some aspects of our lives are more comparable to fungal indeterminancy, he points out. Our daily habits are repetitive, but they are also open-ended, responding to opportunity and encounter. What if our indeterminate life form is not the shape of our bodies but rather the shape of our motions over time? Such indeterminancy expands our concept of human life, showing us how we are transformed by encounter.