The idea of strict causality has long been regarded as the most visible hallmark of modern natural science, and it is indeed a concept through which modern science has taken a long step forward from the science of antiquity. While the ancient Greeks found strict regularity only in mathematics — that is, in a science dealing solely with abstract entities — and contented themselves with merely approximate rules in their knowledge of nature, the modern age has embraced the idea that mathematical strictness is to be found in every natural event, that the "book of nature is written in mathematical symbols", and since the time of Galileo scientists have learned how to implement this idea systematically in the form of a mathematical physics.