The aim of this article is to outline the roles of a negative principle in the Platonic and Aristotelian accounts of biological generation in the Symposium and the Physics, respectively. Specifically, the aim is, on the one hand, to analyse the protagonism of the personification Penia (Poverty) in Socrates’ speech on the progenitors of eros and its intrinsic relationship to the poietic activity of the eroticised animals in the Symposium; and, on the other hand, to examine the Aristotelian notion of privation as the principle of the coming-to-be of new natural beings in Physics. In the first section, it is discussed how the account in the Platonic text deviates from physiology to something more existential: the reproductive impulse corresponds to the feeling of an ontological, primary lack, which is mortality. This is independent of rational consciousness, since animals also exercise poieticactivity. Rationality, exercised at different levels (or steps), only modifies the resource sought to remedy mortality. The emphasis of the Symposium, however, is on the impossibility of reaching the true telos of this needy eros. In the second section, it is analysed how Aristotle, on the other hand, by introducing a rather hermetic systematisation, while nodding to this kind of pothos or mortal nostalgia for an unattainable eternity or divinity, does not give the negative principle of generation, deprivation, a motivational role, whether irrational or rational, in the reproductive impulse. It is matter, as a constitutive aspect of beings, as well as it is responsible for its perishability and potentiality, that is described in a way more similar to Penia. However, the whole of Aristotelian diction is directed precisely towards the attainment of telos in form.