Theophrastus (~371 – ~287 BCE) , a Greek native of Eresos in Lesbos, was the successor to Aristotle in the Peripatetic school. He was vegetarian or vegan, on the grounds that farming animals robs them of life and was therefore unjust. Non-human animals, he said, can reason, sense, and feel just as human beings do.  He seems to have had a naturalistic worldview. He doubted the idea of a spirit independent of organic activity, although stopped short of completely rejecting it.
Theophrastus on Wikipedia