Letter 121 — On Instinct in Animals (§7)

“But people reply: “The reason why animals are so dexterous in the use of their limbs is that if they move them unnaturally, they will feel pain. They are compelled to do thus, according to your school, and it is fear rather than will-power which moves them in the right direction.” This idea is wrong. Bodies driven by a compelling force move slowly; but those which move of their own accord possess alertness. The proof that it is not fear of pain which prompts them thus, is, that even when pain checks them they struggle to carry out their natural motions.”