It is often assumed that one general feature of human agency is that we are and can be responsible not only for things we do but also for things we do not do. The things we do not are often discussed under the general term ‘omission’. In this talk, I shall consider abstinence as Porphyry (c. 235–305 CE) argues for it in his treatise ‘on abstinence from animate creatures’ (translated as On Abstinence from Killing Animals by Gillian Clark, 2000) form the point of view of the notion of omission. Porphyry’s argument is that those philosophers who aim at the highest goal of human life, assimilation to god, to the greatest possible degree, should refrain from not only killing and eating animals but also to causing harm to all living creatures across the board, including plants. I shall ask whether the abstinence Porphyry argues for is an omission in the sense of simple not-doing, or whether we should rather understand it in terms of some positive action that one does when refraining from causing harm to living creatures. I shall argue that abstinence as Porphyry describes it is not merely a simple not-doing but rather should defined by reference to some positive actions or courses of action. In order to explain how I understand Porphyry’s abstinence, I shall also briefly explain how my view differs from that of other scholars on animal rationality and justice.