“That doesn’t follow at all,” said Gladia. “You might well remember knowing it, but be unable to think of it at the moment. I have frequently had something at the tip of my tongue, so to speak, and been unable to retrieve it.”

Daneel said, “I do not understand, madam. If I knew something, surely it would be there when I needed it.”

“Perfect retrieval?” They were walking slowly toward the house.

“Merely retrieval, madam. I am designed so.”

“For how much longer?”

“I do not understand, madam.”

“I mean, how much will your brain hold? With a little over twenty decades of accumulated memories, how much longer can it go on?”

“I do not know, madam. As yet I feel no difficulty.”

“You might not—until you suddenly discover you can remember no more.”

Daneel seemed thoughtful for a moment. “That may be so, madam.”

“You know, Daneel, not all your memories are equally important.”

“I cannot judge among them, madam.”

“Others can. It would be perfectly possible to clean out your brain, Daneel, and then, under supervision, refill it with its important memory content only—say, ten percent of the whole. You would then be able to continue for centuries longer than you would otherwise. With repeated treatment of this sort, you could go on indefinitely. It is an expensive procedure, of course, but I would not cavil at that. You’d be worth it.”

“Would I be consulted on the matter, madam? Would I be asked to agree to such treatment?”

“Of course. I would not order you in a matter like that. It would be a betrayal of Dr. Fastolfe’s trust.”

“Thank you, madam. In that case, I must tell you that I would never submit voluntarily to such a procedure unless I found myself to have actually lost my memory function.”