“The Thinking Animal” explores how American Romantic writers Herman Melville, Emily Dickinson, and Henry David Thoreau perform thought experiments to reveal human thinking’s innate animalistic movement. As these writers probe the enigmatic connection between how a nonhuman animal moves their body – flying, diving, slithering, cantering – and how human thinking moves within the mind, they weave together personal observations of animals and nineteenth-century scientific findings in zoology, mental science, and phenology to trouble traditional representations of human rationality and animal agency. Methodologically, this project operates at the intersection of animal studies, new materialism, posthumanism, and aesthetics by bridging Melville, Dickinson, and Thoreau’s animals with their philosophies of mind and with twenty and twenty-first theories in classification, materiality, and lineology. This multifaceted approach uncovers Melville’s thinking that moves as looping conundrums around scientific classifications of sharks, fish, tortoises, swarms, and even slaves. Dickinson’s thinking moves as a material sensation of expansion, evasion, and creativity prompted by flying birds and weaving spiders. And, Thoreau’s thoughts move seasonally as he simultaneously tracks his thoughts and the movements of the autumn fox, the summer moose, and the winter night owl. Each writer shows how nature’s beings, human and nonhuman, are profoundly connected by the movement of thinking.