Ever since Aristotle proclaimed in the fourth century BCE what was most likely already a widespread assumption: that humans are superior to the other creatures, we have held ourselves in higher esteem than all the rest of creation (though inferior to God and the angels). This conceit was amplified in the 17th century when the influential French philosopher René Descartes reasoned that nonhuman animals—assumed to be lacking thoughts, feelings, and souls—were nothing more than complex machines and therefore ineligible for our moral concern.