‘Launch her,’ called Macintosh. A crane on the gantry swung a raft out over the water and started to lower it. On the raft, gazing at each other sombrely, sat Macintosh’s Principal Research Assistant, Sinson, wearing an inflated yellow life-jacket, and his ethical machine, Samaritan II. A great echoing silence fell over the tank.

Macintosh had concentrated all his department’s efforts on the Samaritan programme. The simplest and purest form of the ethical situation, as he saw it, was the one in which two people were aboard a raft which would support only one of them, and he was trying to build a machine which would offer a coherent ethical behaviour pattern under these circumstances. It was not easy. His first attempt, Samaritan I, had pushed itself overboard with great alacrity, but it had gone overboard to save anything which happened to be next to it on the raft, from seven stone of lima beans to twelve stone of wet seaweed. After many weeks of stubborn argument, Macintosh had conceded that the lack of discrimination in this response was unsatisfactory, and he had abandoned Samaritan I and developed Samaritan II, which would sacrifice itself only for an organism at least as complicated as itself.

The raft stopped, revolving slowly, a few inches above the water.

‘Drop it,’ cried Macintosh.

The raft hit the water with a sharp report. Sinson and Samaritan sat perfectly still. Gradually the raft settled in the water, until a thin tide began to wash over the top of it. At once Samaritan leaned forward and seized Sinson’s head. In four neat movements it measured the size of his skull, then paused, computing. Then, with a decisive click, it rolled sideways off the raft and sank without hesitation to the bottom of the tank.

‘Save it, Lord,’ boomed Macintosh to a young man waiting on the side of the tank in a swimming costume. Lord dived in and attached a rope to the sunken Samaritan.

‘Why don’t you tie the rope to it before it goes overboard?’ asked Goldwasser.

‘I don’t want it to know that it’s going to be saved. It would invalidate its decision to sacrifice itself.’

‘But how would it know?’

‘Oh, these Samaritan IIs are canny little beggars. Sometimes I think they understand every damned word you say to them.’

‘They’re far too simple, Macintosh . . .’

‘No, no. They come to trust you. So every now and then I leave one of them in instead of fishing it out. To show the others I mean business. I’ve written off two this week.’

Samaritan was being hauled out on to the side of the tank, and held upside down to drain the water out of its works. Every now and then it ticked faintly, and shuddered.

‘You’ve heard the news about your new wing, have you?’ said Goldwasser.

‘What news?’

‘About the Queen.’

‘No?’

‘She’s coming down to open it.’

‘Really?’

He leaned over the rail and shouted: ‘Oh, Lord! Is it all right? Load it up for the next run, then.’