"Jimmy looked at Robutt, who was squeking again, a very low, slow squeak, that seemed frightened. Jimmy held out his arms and Robutt was in them in one bound. Jimmy said, 'What will the difference be between Robutt and the dog?'
'It's hard to explain,' said Mr. Anderson, 'but it will be easy to see. The dog will really love you. Robutt is just adjusted to act as though it loves you.'
'But, Dad, we don't know what's inside the dog, or what his feelings are. Maybe it's just acting, too.'"
A Boy's Best Friend, Isaac Asimov