Why do some religions forbid harming any living thing?

Jainism, Hinduism and Buddhism share a principle called ahimsa, non-harming. The Sanskrit word is simply "not" plus "harm": hiṃsā means injury, a-hiṃsā its opposite. In these traditions, harming another living being is a moral wrong that affects one's own future, including future rebirths. Ahimsa is the first of Jainism's five great vows, a central precept of Hinduism, and the first of Buddhism's five precepts.

Jainism takes it furthest. Jains classify living beings by their senses, from one-sensed beings such as plants and even water and earth, up to humans and animals. Jain ascetics sweep the ground in front of them so as not to crush tiny creatures, and strict Jains avoid root vegetables like potatoes and onions.

The same ethic helps explain why the cow is sacred to many Hindus. It wasn't always so: the oldest Vedic texts mention animal sacrifice, and there is evidence of beef eating in ancient India. But the Rigveda already calls dairy cows aghnya, "not to be slaughtered", and by about 1000 CE vegetarianism and a beef taboo were mainstream Hindu tradition. Today India has more vegetarians than the rest of the world combined.

In the 20th century, Mohandas Gandhi turned this ancient religious principle into a tool of mass political action. His non-violent resistance influenced civil rights leaders far beyond India, including Martin Luther King Jr.