"But the greatest of all the prejudices we have retained from infancy is that of believing that brutes think." (Rene Descartes, 1649)
"When a Creature has given such convincing and undeniable Proofs of the Terrors upon him, and the Pains and Agonies he feels, is there a Follower of Descartes so inur’d to Blood, as not to refute, by his Commiseration, the Philosophy of that vain Reasoner?" (Bernard Mandeville, 1723)
Rene Descartes (1596-1650), "the father of modern philosophy," believed that humans have two components: body and mind. Most of what we do can be explained by unthinking responses, or "passions" to our environment. But animals, lacking mind, act and interact through passions only. They are, in short, organic "automata" (machines), "much more splendid than artificial ones," but machines nonetheless.
For Descartes, the immaterial mind is manifested by the use of creative language, and creatively using language is exclusive to humans. To be sure, animals communicate, but they are merely relaying instinctive desires (food, water) or demonstrating involuntary reactions (pain, excitement). In other words, they are conscious but not self-conscious. They cannot think and then convey, "Boy, this hurts" or "I wish they would stop doing this to me." And so, animal pain, because it is not true pain, is morally irrelevant. It simply does not matter. Rene Descartes had given man the sanction to brutalize with impunity.
Gary Francione (Introduction to Animal Rights): "Descartes and his followers performed experiments in which they nailed animals by their paws onto boards and cut them open to reveal their beating hearts. They burned, scalded, and mutilated animals in every conceivable manner. When the animals reacted as though they were suffering pain, Descartes dismissed the reaction as no different from the sound of a machine that was functioning improperly. A crying dog, Descartes maintained, is no different from a whining gear that needs oil."
360 years later, Rene Descartes still exerts influence. While the existence of animal pain is beyond reasonable debate, many still see that pain (suffering) as different from ours, and this, to some, justifies animal experimentation. Yet clearly, no two humans suffer in precisely the same way; I can never truly know what pain feels like to you. I can intuit by virtue of a similar anatomy - nociceptors, spinal cord, brain - but this anatomy is one we also share with animal test subjects; it's why we experiment on them in the first place. Because of his time and place, Descartes' ignorance (and cruelty) can (almost) be excused. Today, we know better. The machines feel. It's time we stopped hurting them.