Before the invention or discovery of the concept of "nature" (Ancient Greek phusis), by the Pre-Socratic philosophers, the same words tend to be used to describe the natural "way" in which a plant grows,[12] and the "way" in which, for example, one tribe worships a particular god. For this reason it is claimed these men were the first philosophers in the strict sense, and also the first people to clearly distinguish "nature" and "convention".[13] Science was therefore distinguished as the knowledge of nature, and the things which are true for every community, and the name of the specialized pursuit of such knowledge was philosophy — the realm of the first philosopher-physicists. They were mainly speculators or theorists, particularly interested in astronomy. In contrast, trying to use knowledge of nature to imitate nature (artifice or technology, Greek technē) was seen by classical scientists as a more appropriate interest for lower class artisans.[14] A clear-cut distinction between formal (eon) and empirical science (doxa) was made by pre-Socratic philosopher Parmenides (fl. late sixth or early fifth century BCE). Although his work peri physeos is a poem, it may be viewed as an epistemological essay, an essay on method in natural science. Parmenides' ἐὸν may refer to a formal system, a calculus which can describe nature more precisely than natural languages.