text: 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. Thus the child who is trying to stand and is becoming used to carry his own weight, on beginning to test his strength, falls and rises again and again with tears until through painful effort he has trained himself to the demands of nature. And certain animals with hard shells, when turned on their backs, twist and grope with their feet and make motions side-ways until they are restored to their proper position. The tortoise on his back feels no suffering; but he is restless because he misses his natural condition, and does not cease to shake himself about until he stands once more upon his feet.