Contained in Plato's The Republic is the account of Socrates', debate with Glaucon, Plato's elder brother, on the issue that meat eating was inhumane, unethical, unhealthy, and would lead to unhappiness, war and social justice.
Socrates: Would this habit of eating animals not require that we slaughter animals that we knew as individuals, and in whose eyes we could gaze and see ourselves reflected, only a few hours before our meal?
Glaucon: This habit would require that of us.
Socrates: Wouldn't this [knowledge of our role in turning a being into a thing] hinder us in achieving happiness?
Glaucon: It could so hinder us in our quest for happiness.
Socrates: And, if we pursue this way of living, will we not have need to visit the doctor more often?
Glaucon: We would have such need.
Socrates: If we pursue our habit of eating animals, and if our neighbor follows a similar path, will we not have need to go to war against our neighbor to secure greater pasturage, because ours will not be enough to sustain us, and our neighbor will have a similar need to wage war on us for the same reason?
Glaucon: We would be so compelled.
Socrates: Would not these facts prevent us from achieving happiness, and therefore the conditions necessary to the building of a just society, if we pursue a desire to eat animals?
Glaucon: Yes, they would so prevent us.
Quoted by Plato in The Republic
Socrates (469 BC-399 BC), Greek philosopher