The great moral philosopher Jeremy Bentham, founder of utilitarianism, famously said,'The question is not, "Can they reason?" nor, "Can they talk?" but rather, "Can they suffer?" Most people get the point, but they treat human pain as especially worrying because they vaguely think it sort of obvious that a species' ability to suffer must be positively correlated with its intellectual capacity. Plants cannot think, and you'd have to be pretty eccentric to believe they can suffer. Plausibly the same might be true of earthworms. But what about cows?
What about dogs? I find it almost impossible to believe that René Descartes, not known as a monster, carried his philosophical belief that only humans have minds to such a confident extreme that he would blithely spreadeagle a live mammal on a board and dissect it. You'd think that, in spite of his philosophical reasoning, he might have given the animal the benefit of the doubt. But he stood in a long tradition of vivisectionists including Galen and Vesalius, and he was followed by William Harvey and many others (See from which this picture is taken).
How could they bear to do it: tie a struggling, screaming mammal down with ropes and dissect its living heart, for example? Presumably they believed what came to be articulated by Descartes: that non-human animals have no soul and feel no pain.