"Because it puts two identical personalities on one plane." The man with Horning's face dropped into a chair and hunched forward. "Take our own situation as an example. You're married to a shrew, a termagant. You want to leave her." "Yes." "I, on the other hand, have a young and charming wife who holds a considerable fortune in her own right. Consequently, it would be ever so much to your advantage to switch places with me." Horning's counterpart brought up one square-knit hand in an expressive gesture. "What's to prevent your murdering me and moving in?" Horning nodded slowly. "I see what you mean." "I'm convinced it's actually happened a few times already," the other asserted. "Though of course it's not generally known. Fortunately, we've never worked out the principle on this plane." He paused to drink, then set down his glass. His eyes narrowed thoughtfully, and he nodded in the direction of the transdimensional registration unit. "Just how does it work, Doctor? I've always wondered where my own experiments went wrong." For a moment Horning hesitated, then shrugged. "See for yourself." Kneeling, he unsnapped the unit's back plate and exposed the circuits. "The registration dials are set with my own world as zero. You pick up others in the scanning scope as you go, within the limits of the projector drive. After that, it's just a problem of reintegration." Beside him, the man who was his coexisting self craned. "So that's it! I never dreamed it could be so simple." "I used a light-loop to help break through the barrier," Horning explained, sketching out a hasty diagram. "It helps to increase the power output--" "Of course." The other was down on the floor now, probing into the unit's workings. "I've developed all the component elements at one time or another, but when it came to combining them properly, I always managed to miss out." * * * * * Horning rose and drained his glass. "Well, you know now," he observed. "For my part, I'm ready to start work on some other project, now that I've gotten to this world." "I was afraid you'd say that," the other Doctor Raymond X. Horning remarked. Straightening, he snapped shut the back panel of the transdirectional registration unit. "But ... it's not easy." "What do you mean?" Horning's counterpart got up. "I mean you can't stay in this world. You're going to have to leave again." "To leave--!" Horning turned sharply. "Yes." Beneath the blandness of the other's manner, a new note rang, grim and unyielding. "As I pointed out, interdimensional transit's a logical impossibility. There's no way of integrating two identical personalities, two selves of the same man, into a highly organized society such as this one." "And for a reason like that you'd try to force me out--?" Horning took an angry step forward. But his counterpart jumped back, out of the way. His hand darted to his pocket, whipping out the snub-nosed pistol. Horning came to an abrupt halt.