Heavenly signs. Dutch conceptions of the comet of 1618 The seventeenth-century fear for comets is, since the enlightenment, generally considered as a 'superstition' due to ignorance about the workings of nature. A closer examination of the tracts devoted to the great comet of 1618 by the famous Dutch natural philosophers Snellius and Mulerius, the well-known theologian Maccovius, and the popular poet Cats shows that the observation of comets played a significant role in the proof of the existence of God, and so, did not involve superstition at all. The idea that comets, like other portents, were divine warnings, sprang from a coherent view of the world as a moral order reflecting God's purposes, and physically sensitive to the moral conduct of humanity. It is almost impossible to make a distinction between 'science' and 'religion' in early seventeenth- century Calvinism, since the study of nature in general, and of the miracles of nature in particular, was a study of the works of God.