This thesis positions Thoreau’s texts alongside theories of materialist-determinist and indeterminist causality that developed from the causal debate of Enlightenment and nineteenth-century philosophy and science. These debates frame Thoreau’s progression from an epistemological Baconian-Newtonian inductionism to an ontological New Materialist vitalism in his observations of natural causation. As Thoreau traces relations back to possible causes, his language shifts from empirical observations to imaginative inductions and inferences that attempt to complete insoluble causal equations. For Thoreau, thinking about causality in these terms leads to an equivocation of ontological boundaries between human and nonhuman where Thoreau’s mental and physical actions are inseparable from the motions of nonhuman agents. Examining causal events in Thoreau’s texts provides a new interpretation that views theories of motion and causality as central to his epistemology and ontology.