The article proposes to re-examine the Lucretian formula naturae species ratioque (1.146–148 = 2.59–61 = 3.91–93 = 6.39–41), the meaning of which has prompted some critical debate. The examination begins from an analysis of rhetoric and argument in the sections in which the phrase occurs, with the goal of demonstrating that the meaning ‘rational vision of nature’ is more apt to the context and to Lucretius’ poetic and philosophical programme, which often relies on metaphors drawn from the semantic field of (...) vision to describe the comprehension of natural phenomena and the didactic aim of the work. In the light of this, the final part of the paper discusses the textual problem concerning lines 6.56–57 (= 90–91), which are normally considered spurious but which can be understood better in the light of the Lucretian conception of philosophy (and of poetry) as penetrating vision. (shrink)