On Abstinence from Eating Animals (Koine Greek: Περὶ ἀποχῆς ἐμψύχων, romanized: Peri apochēs empsychōn, Latin: De abstinentia ab esu animalium) is a 3rd-century treatise by Porphyry on the ethics of vegetarianism. The four-book treatise was composed by the philosopher as an open letter to Castricius Firmus, a fellow pupil of Plotinus who had renounced a vegetarian diet.

De abstinentia is the most detailed surviving work discussing vegetarianism from classical antiquity. Porphyry advocates for vegetarianism on both spiritual and ethical grounds, applying arguments from his own school of Neoplatonism to counter those in favour of meat-eating from the Stoic, Peripatetic, and Epicurean schools. Porphyry argues that there is a moral obligation to extend justice to animals because they are rational beings. He discusses societies that have been historically vegetarian, the implications of metempsychosis (transmigration of the soul), and offers arguments against animal sacrifice. Porphyry directs his discourse towards philosophers, and does not advocate that people such as soldiers or athletes adopt a vegetarian diet.

According to philosopher Daniel Dombrowski, in De abstinentia Porphyry originated the argument from marginal cases, that is, that if animals are not afforded moral status, then neither should "marginal cases" of human beings such as infants, persons with severe cognitive disabilities, and the senile.

The treatise is written in Koine Greek but is often referred to in academia by the abbreviation of its Latin name, De abstinentia. While the manuscript traditions of the text seem to faithfully represent Porphyry's ideas and arguments, they contain errors and lack fidelity to the original. The entirety of the work is extant except for the ending of the fourth book.

Porphyry was born c. 234 in Tyre, Roman Phoenicia. He studied under Plotinus and promulgated the philosophy of the Neoplatonists. He composed original works in Greek and wrote On Abstinence from Eating Animals between 263 and 301, most probably between the years 268 and 271 when he was living in Sicily.

Porphyry composed De abstinentia as an extended open letter to Castricius Firmus, a fellow pupil of Plotinus who had renounced vegetarianism and began eating meat after converting to a Peripatetic philosophy, possibly around 270. Castricius Firmus did not find a vegetarian diet to be justifiable in theory and found that it was irreconcilable with the demands of public life and established religion. He made public statements to explain his attitude and offered arguments to justify a meat-eating diet.

As a treatise that collects various arguments for and against vegetarianism, De abstinentia is evidence of the existence of a wider debate on the subject during Porphyry's times. It follows other works from classical antiquity that address vegetarianism and animal ethics, including Plutarch's Odysseus and Gryllus, On Animal Cleverness, and On the Eating of Flesh, all of which were collected in the Moralia. In addition to Plutarch, several of Porphyry's philosophical predecessors were known to be or thought to have been vegetarian, including Pythagoras, Seneca the Younger, Empedocles, Theophrastus, and Ovid.

In Porphyry's four-book treatise, he introduces various arguments to induce Castricius Firmus to return to a vegetarian diet. De abstinentia includes a polemical review of arguments both for and against vegetarianism. It draws on quotations, empirical reports, and arguments derived from other authors. Many of the arguments that Porphyry raises are rebuttals to arguments for meat-eating that were common to his era. The rationales offered for adopting a vegetarian diet are quite varied, and include beliefs in the transmigration of the soul, that meat-eating is detrimental to the soul or the body, and that it causes unnecessary suffering in animals. The treatise raises the philosophical questions of whether non-human animals are rational, whether they have souls, and how the concept of justice should then be applied to them.