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Soot is known for its enormous and pervasive negative impacts on human health and the environmental, but there much about soot that is not well known, including the precursors and chemical mechanisms involved in its formation. Many studies have characterized species associated with incipient particles, i.e., the first particles produced during soot formation. These studies provide insight into inception mechanisms, the pathways leading from gas-phase precursors to condensed-phase particles. Potential inception mechanisms involve one (or a combination) of two classes of pathways: physical nucleation, in which precursors undergo a thermodynamic phase change and are bound together by electrostatic forces, and chemical clustering, in which precursors react to form covalently bound clusters. In a recent paper, Shao et al.1 concluded that soot inception occurs through physical nucleation and claimed to have provided direct evidence of such a mechanism. We demonstrate that this conclusion is inconsistent with (1) the consensus of published work, (2) the data, theory, and analysis on which this conclusion is nominally based, and (3) the second law of thermodynamics. We show that, contrary to their conclusions, their experimental and theoretical results provide evidence for a chemical-clustering soot-inception mechanism.