General Kallner spoke quickly. 'I didn't realize that you were unaware of this situation, Dr Calvin. I needn't tell you that on Earth many people strongly oppose robots. The government, of course, argues that robots always have an unbreakable First Law. But we desperately needed robots of a different nature. So just a few NS-2s were made for us without the whole of the First Law. To keep the secret, all NS-2s are made without serial numbers, and those without the whole of the First Law are delivered with a group of normal robots.'