Pythagoras

circa 582-507 B.C.
Greek philosopher, mathematician, mystic, known as the Father of Vegetarianism.
 
As long as men massacre animals, they will kill each other. Indeed, he who sows the seeds of murder and pain cannot reap the joy of love.

The animals share with us the privilege of having a soul

Alas, what wickedness to swallow flesh into our own flesh, to fatten our greedy bodies by cramming in other bodies, to have one living creature fed by the death of another! In the midst of such wealth as earth, the best of mothers, provides, yet nothing satisfies you, but to behave like the Cyclopes, inflicting sorry wounds with cruel teeth! You cannot appease the hungry cravings of your wicked, gluttonous stomachs except by destroying some other life.

As long as man continues to be the ruthless destroyer of lower living beings he will never know health or peace. For as long as men massacre animals, they will kill each other. Indeed, he who sows the seed of murder and pain cannot reap joy and love.

Hippocrates 
460 BC – ca. 370 BC was an ancient Greek physician and is considered one of the most outstanding figures in the history of medicine. He is referred to as the " father of medicine"

The soul is the same in all living creatures, although the body of each is different.

Seneca
C.5 - C.E.65
Roman Stoic philosopher, statesman, dramatist, and in one work humorist, of the Silver Age of Latin literature. He was tutor and later advisor to emperor Nero.

But for the sake of some little mouthful of meat,
we deprive a soul of the sun and light,
and of that proportion of life and time it had been
born into the world to enjoy.

If true, the Pythagorean principles as to abstain from flesh, foster innocence; if ill-founded they at least teach us frugality, and what loss have you in losing your cruelty? It merely deprives you of the food of lions and vultures…let us ask what is best - not what is customary. Let us love temperance - let us be just - let us refrain from bloodshed.

Plato 
428-347 B.C.
Born in Athens, Greece, he is the best known of the ancient philosophers and helped to establish the foundations of Western philosophy.

The gods created certain kinds of beings to replenish our bodies...
they are the trees and the plants and the seeds.

Plutarch
AD 46 – 120
Greek historian, philosopher, essayist and biographer. He authored a number of treatises on matters of ethics on topics such as education, marriage, religious observances and also upon the status of animals which included discussion on the presence of reason in non human animals and the practice of ethical vegetarianism. Of particular note is his essay:  On the Eating of Animal Flesh, Book 12, The Moralia

Can you really ask what reason Pythagoras had for abstaining from flesh? For my part I rather wonder both by what accident and in what state of soul or mind the first man did so, touched his mouth to gore and brought his lips to the flesh of a dead creature, he who set forth tables of dead, stale bodies and ventured to call food and nourishment the parts that had a little before bellowed and cried, moved and lived. How could his eyes endure the slaughter when throats were slit and hides flayed and limbs torn from limb? How could his nose endure the stench? How was it that the pollution did not turn away his taste, which made contact with the sores of others and sucked juices and serums from mortal wounds?… It is certainly not lions and wolves that we eat out of self-defense; on the contrary, we ignore these and slaughter harmless, tame creatures without stings or teeth to harm us, creatures that, I swear, Nature appears to have produced for the sake of their beautyand grace. But nothing abashed us, not the flower-like tinting of the flesh, not the persuasiveness of the harmonious voice, not the cleanliness of their habits or the unusual intelligence that may be found in the poor wretches. No, for the sake of a little flesh we deprive them of sun, of light, of the duration of life to which they are entitled by birth and being.

If you declare that you are naturally designed for such a diet, then first kill for yourself what you want to eat. Do it, however, only through your own resources, unaided by cleaver or cudgel or any kind of ax

The obligations of law and equity reach only to mankind;
but kindness and beneficence should be extended
to the creatures of every species,
and these will flow from the breast of a true man,
as streams that issue from the living fountain.

Note that the eating of flesh is not only physically against nature, but it also makes us spiritually coarse and gross by reason of satiety and surfeit.

Bion of Borysthenes,
325-c. 250 BC
was a Greek philosopher. After being sold into slavery, and then released, he moved to Athens, where he studied in almost every school of philosophy available.

Though boys throw stones at frogs in sport, the frogs do not die in sport, but in earnest.
Bion, Water and Land Animals,"

Socrates
469 BC–399 BC
was a Classical Greek philosopher and is credited as one of the founders of Western philosophy.

Contained in Plato’s 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. The following is a most compelling argument

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

Ancient Chinese verse

For hundreds of thousands of years the stew in the pot
Has brewed hatred and resentment that is difficult to stop.
If you wish to know why there are disasters of armies and weapons in the world,
Listen to the piteous cries from the slaughter house at midnight.

Moses Maimonides

Born in Cordoba, Spain on March 30, 1135, and died in Egypt on December 13, 1204, was a preeminent medieval Jewish philosopher and one of the greatest Torah scholars of the Middle Ages. He worked as a rabbi, physician, and philosopher in Spain, Morocco and Egypt

There is no difference between the pain of humans and the pain of other living beings, since the love and tenderness of the mother for the young are not produced by reasoning, but by feeling, and this faculty exists not only in humans but in most living beings.
Guide for the Perplexed

It should not be believed that all beings exist for the sake of the existence of man. On the contrary, all the other beings too have been intended for their own sakes and not for the sake of anything else.
Guide for the Perplexed

Voltaire
1694 – 1778  was a French Enlightenment writer, essayist, and philosopher known for his wit and his defence of civil liberties, including both freedom of religion and free trade.

People must have renounced, it seems to me, all natural intelligence to dare to advance that animals are but animated machines.... It appears to me, besides, that [such people] can never have observed with attention the character of animals, not to have distinguished among them the different voices of need, of suffering, of joy, of pain, of love, of anger, and of all their affections. It would be very strange that they should express so well what they could not feel.

Jean-Jacques Rousseau
Jean Jacques Rousseau  1712- 1778 born in Geneva Switzerland was a French philosopher, writer, and composer of the eighteenth-century Enlightenment, whose political philosophy influenced the French Revolution and the development of modern political and educational thought.

The animals you eat are not those who devour others; you do not eat the carnivorous beasts, you take them as your pattern. You only hunger for the sweet and gentle creatures which harm no one, which follow you, serve you, and are devoured by you as the reward of their service.

Jeremy Bentham
1748 – 1832 born in Spitalfields, London was a English jurist, philosopher, and legal and social reformer.

The question is not, "Can they reason?" nor, "Can they talk?" but rather, "Can they suffer?"

Immanuel Kant
1724–1804 German philosopher was one of the most influential philosophers in the history of Western philosophy. His contributions to metaphysics, epistemology, ethics, and aesthetics have had a profound impact on almost every philosophical movement that followed him

cruelty to animals is contrary to man's duty to himself, because it deadens in him the feeling of sympathy for their sufferings, and thus a natural tendency that is very useful to morality in relation to other human beings is weakened.

If [man] is not to stifle his human feelings, he must practise kindness towards animals, for he who is cruel to animals becomes hard also in his dealings with men. We can judge the heart of a man by his treatment of animals

Arthur Schopenhauer
1788 - 1860 was a German Philosopher known for his atheistic pessimism and philosophical clarity. At age 25, he published his doctoral dissertation, On the Fourfold Root of the Principle of Sufficient Reason

The assumption that animals are without rights and the illusion that our treatment of them has no moral significance is a positively outrageous example of Western crudity and barbarity. Universal compassion is the only guarantee of morality.

Henry David Thoreau
Henry David Thoreau, 1817– 1862, was an American author, poet, naturalist, surveyor, historian, philosopher, and leading transcendentalist. He is best known for his book Walden, a reflection upon simple living in natural surroundings, and his essay, Civil Disobedience, an argument for individual resistance to civil government in moral opposition to an unjust state.

I have no doubt that it is a part of the destiny of the human race, in its gradual improvement, to leave off eating animals, as surely as the savage tribes have left off eating each other when they came in contact with the more civilized.
Walden: Life in the woods

No humane being, past the thoughtless age of boyhood, will wantonly murder any creature which holds its life by the same tenure that he does.

I saw a muskrat come out of a hole in the ice ... While I am looking at him, I am thinking what he is thinking of me. He is a different sort of man, that's all.

The squirrel that you kill in jest, dies in earnest.

Every man who has ever been earnest to preserve
his higher or poetic faculties in the best condition,
has been particularly inclined to abstain from animal food

I have no doubt that it is a part of the destiny of the human race, in its gradual improvement, to leave off eating animals, as surely as the savage tribes have left off eating each other...."

I have just been through the process of killing a cistudo for the sake of science; but I cannot excuse myself for this murder, and see that such actions are inconsistent with the poetic perception, however they may serve science, and will affect the quality of my observations. I pray that I may walk more innocently and serenely through nature. No reasoning whatever reconciles me to this act. It affects my day injuriously. I have lost some self-respect. I have a murderer's experience to a degree.

One farmer says to me, 'You cannot live on vegetable food solely, for it furnishes nothing to make the bones with;' and so he religiously devotes a part of his day to supplying himself with the raw material of bones; walking all the while he talks behind his oxen, which, with vegetable-made bones, jerk him and his lumbering plow along in spite of every obstacle.

Leo Tolstoy,
Count Leo Nikolayevich Tolstoy, 1828 - 1910 is best known as one of Russias greatest novelists, most well know of his novels being War and peace and Anna Karenina. He was also a contemporary of Russian writer Fyodor Dostoyevsky another advocate of vegetarianism and animal rights. Tolstoy was however more than a writer he was a moral philosopher, a humanitarian, a mystic.

A man can live and be healthy without killing animals for food; therefore, if he eats meat, he participates in taking animal life merely for the sake of his appetite. And to act so is immoral.

As long as there are slaughterhouses, there will be battlefields

If a man earnestly seeks a righteous life,
his first act of abstinence is from animal food

What I think about vivisection is that if people admit that they have the right to take or endanger the life of living beings for the benefit of many, there will be no limit to their cruelty."

A man can live and be healthy without killing animals for food; therefore, if he eats meat, he participates in taking animal life merely for the sake of his appetite. And to act so is immoral.

"'Thou shalt not kill' does not apply to murder of one's own kind only, but to all living beings; and this Commandment was inscribed in the human breast long before it was proclaimed from Sinai.

Bertrand Russell
Bertrand Arthur William Russell, 1872 – 1970 Trellech, Monmouthshire was a British philosopher, logician, mathematician, historian, socialist, pacifist and social theorist.

"It has been said that man is a rational animal. All my life I have been searching for evidence which could support this.

Albert Schweitzer 1875-1965 medical missionary, philosopher, musician and Nobel peace prize winner in 1953 for his philosophy of "Reverence for Life".

Very little of the great cruelty shown by men can really be attributed to cruel instinct.  Most of it comes from thoughtlessness or inherited habit.  The roots of cruelty, therefore, are not so much strong as widespread.  But the time must come when inhumanity protected by custom and thoughtlessness will succumb before humanity championed by thought.  Let us work that this time may come. 

Think occasionally of the suffering of which you spare yourself the sight.

Sri Aurobindo 1872-1950 was an Indian nationalist and freedom fighter, poet, philosopher, and yogi. Between 1905 to 1910 he joined the movement for India's freedom from British rule . He developed his own philosophy of human progress and spiritual evolution.

Life is life - whether in a cat, or dog or man. There is no difference there between a cat or a man. The idea of difference is a human conception for man's own advantage.
 

Mohandas Gandhi
Mohandas Karamchand Gandhi 2 October 1869 - 1948 was born in Porbandar, a coastal town in present-day Gujarat, India. He was the pre-eminent political and spiritual leader of India. He was commonly called by the honorific Mahatma Gandhi, which means great soul. Best known for his non violent struggle for India's freedom during the Indian independence movement.

I abhor vivisection with my whole soul.  All the scientific discoveries stained with innocent blood I count as of no consequence. 

It is very significant
that some of the most
thoughtful and cultured men
are partisans of a pure vegetable diet

The greatness of a nation and its moral progress can be judged by the way its animals are treated

I do feel that spiritual progress does demand at some stage that we should cease to kill our fellow creatures for the satisfaction of our bodily wants.

It is necessary to correct the error that vegetarianism has made us weak in mind, or passive or inert in action. I do not regard flesh-food as necessary at any stage

I still believe that man, not having been given the power of creation, does not posses the right of destroying the meanest creature that lives. The perogative of destruction belongs solely to the Creator of all that lives.

To my mind, the life of a lamb is no less precious than that of a human being. I should be unwilling to take the life of a lamb for the sake of the human body.

Complete non-violence is complete absence of ill-will against all that lives. It therefore embraces even sub-human life, not excluding noxious insects and beasts. They have not been created to feed our destructive propensities. If we only knew the mind of the Creator, we should find their proper place in His creation.

Pete Singer
Peter Albert David Singer, born 6 July 1946, is an Australian philosopher. He is best known for his book Animal Liberation published in 1995 widely regarded as the standard of the animal liberation movement.

In an earlier stage of our development most human groups held to a tribal ethic. Members of the tribe were protected, but people of other tribes could be robbed or killed as one pleased. Gradually the circle of protection expanded, but as recently as 150 years ago we did not include blacks. So African human beings could be captured, shipped to America and sold. In Australia white settlers regarded Aborigines as a pest and hunted them down, much as kangaroos are hunted down today. Just as we have progressed beyond the blatantly racist ethic of the era of slavery and colonialism, so we must now progress beyond the speciesist ethic of the era of factory farming, of the use of animals as mere research tools, of whaling, seal hunting, kangaroo slaughter and the destruction of wilderness. We must take the final step in expanding the circle of ethics.

Because one species is more clever than another, does it give it the right to imprison or torture the less clever species? Does one exceptionally clever individual have a right to exploit the less clever individuals of his own species? To say that he does is to say with the Fascists that the strong have a right to abuse and exploit the weak - might is right, and the strong and ruthless shall inherit the earth."