Read "Second Stage Lensmen" by E.E. Smith online for free on Textopian. Full text with search, annotations, highlights, and AI-powered reading aids.
"But listen, Kim! " Clarrissa protested. "What makes you all so sure that it's Overlords? There's nothing on my map there to prove -- Why, it might be _anything_!"
"It might not, too," Kinnison stated. "Barring the contingency of the existence of a life form unknown to any one of the four of us and which operates exactly as the Overlords do operate, that hypothesis is the only one both necessary and sufficient to explain all the facts which you have plotted upon your chart. Think a minute -- you know how they work. They tune in on some one mind, the stronger and more vital the better. The fact that the Lyranians have such powerful minds is undoubtedly one big reason why the Overlords are here. In that connection, it's a mystery to me how Helen has lived so long -- all the persons who disappeared had high-powered minds, didn't they?"