<P> Throughout history individuals have condemned various practices that they considered to be cruel to animals . Renaissance polymath Leonardo da Vinci, for example, was troubled by the sight of birds in captivity, and (according to biographer Giorgio Vasari) once purchased caged birds in order to set them free . He also expressed anger within his notebooks with the fact that humans use their strength and power to raise animals for slaughter . </P> <P> René Descartes contrarily believed that non-humans are automata, complex machines with no soul, mind, or reason . In Cartesian dualism, consciousness was unique to human among all other animals and linked to physical matter by divine grace . However, close analysis shows that many human features such as complex sign usage, tool use, and self - consciousness can be found in some animals . </P> <P> Charles Darwin, by presenting the theory of evolution, revolutionized the way that humans viewed their relationship with other species . Darwin believed that not only did human beings have a direct kinship with other animals, but the latter had social, mental and moral lives too . Later, in The Descent of Man (1871), he wrote: "There is no fundamental difference between man and the higher mammals in their mental faculties ." </P> <P> Some philosophers and intellectuals, such as Peter Singer and Tom Regan, have argued that animals' ability to feel pain as humans do makes their well - being worthy of equal consideration . There are many precursors of this train of thought . Jeremy Bentham, the founder of utilitarianism, famously wrote in his An Introduction to the Principles of Morals and Legislation (1789): </P>

What is the charge for killing an animal