Where are the edges of a tree? What makes arms different from daughters? What have corals got in common with Necker cubes? Biological individuality has become a dizzying area since researchers began, at the turn of the millennium, to see new connections between group selection and the evolution of multicellularity and to realize that it isn't feasible to go on taking organisms for granted as basic particles of the living world. Ellen Clarke argues in this ambitious and disorienting romp through the natural world that our way of conceptualizing living things—of understanding the living world as carved up into numerous separate chunks—is best understood as an idealization. This idealization serves valuable pragmatic and theoretical purposes, but stands as a distortion nonetheless of the more messy and variable reality. Vivid biological examples and lively prose are used to animate some fairly arcane philosophical topics concerning identity over time, natural kinds, and the fundamental furniture of reality, as well as serious biological issues concerning evolutionary theory, the emergence of compositional biological hierarchies, and the evolution of cooperation. The reader will come away with newfound awe and respect for humankind's surprising ingenuity in engineering concepts that make sense of the complex and ever-changing wonders of life on earth.