It is here argued that Halley's comet had a more pivotal role than has hitherto been believed in triggering Newton's acceptance of the law of gravity, dispelling his belief in Descartes' theory of vortices. It is found that historians have been unduly prone to credit Newton with dynamical insights at an earlier date than is warranted by the historical documents. A more convincing account of the transition from the period of Newton's alchemical researches of the 1670s to that of his (...) dynamical insights in the 1680s thereby becomes feasible. Acceptance of a principle of rectilinear inertia was a precondition for formulating the law. (shrink)