It's a coiled-spring, trigger-mechanism kind of impulse and neither the man nor the woman have to be attracted to each other on the personality level, unless you want to be technical and regard the purely physical as an attribute of personality. They can be young or old, plain or good looking. Some attraction will be present, even under the most adverse circumstances. But when the woman is young and beautiful and the personality level warm and appealing you'll be deceiving yourself if you think the impulse can be kept from arising just because you already have a mate you're desperately in love with. You can conquer the impulse if you try hard enough and your love for someone else is strong enough. That's what is meant by loyalty. But you can't keep the impulse from arising and it makes no sense at all to feel guilty about it. The human brain is a resourceful instrument and there are a dozen ways of keeping a tight grip on your nerves when you wake up on a hospital cot and hear unfamiliar voices talking about you. I chose the way that was most natural to me. I concentrated on the scientific construct I've just summarized, letting my mind glide over, and play around with it for a minute or two and telling myself that I must thank the nurse for all that she had done for me. When Gruff Voice left there would be a glow, a brief moment of warmth between us that might have become a high-leaping flame if I hadn't been in love with Joan and she hadn't been carrying a torch for Gruff Voice. I wasn't even sure she was beautiful, but it seemed likely, because you can tell a great deal about a woman just from the sound of her voice. Even if she bent over and kissed me, her eyes shining a little because she'd helped me outdistance Death a yard from the finish line and was feeling grateful and thrilled about it ... well, that would have been all right too. I didn't think Joan or the man who had just taken her into his arms would have held that kind of kiss against us. I had the feeling that Gruff Voice was a generous-minded, all right guy, and if an operation had been necessary to save my life he'd done his best to increase my chances with all of the surgical know-how at his command. Just that thought made me decide to open my eyes and try to raise myself a little, because he had a right to know how grateful I felt. He was just going through the door. I could see that he was tall, blond and rather sturdily built, but a wave of dizziness made me sink back against the pillows again before I could get a really good look at him. It's hard to tell what a man looks like anyway, when he's facing away from you, and you can only see his disappearing shoulders and the back of his head.