carbide-Welsbach lamp. The shutters blinked, sending pulses of light down the length of the ten-foot diameter glass-walled tube in which the men were working. Far down the tube, MacHeath could see the answering flicker from Harry, a mile and a half away in the darkness.

MacHeath watched the screen again. After a few seconds, he said: "O.K.! Hold it!"

Again the lamp flashed.

"Well, it isn't perfect," MacHeath said, "but it's all we can do from here. We'll have to evacuate the tube to get her in perfect balance. Tell Harry to knock off for the day."

While the welcome message was being flashed, MacHeath shut off the testing instruments and disconnected them. It was possible to compensate a little for the testing equipment, but a telephone, or even an electric flashlight, would simply add to the burden.

Bill Griffin shoved down the key on the lamp he was holding and locked it into place. The shutters remained open, and the lamp shed a beam of white light along the shining walls of the cylindrical tube. "How much longer do you figure it'll take, Dave?" he asked.

"Another shift, at least," said MacHeath, picking up the compact, shielded instrument case. "You want to carry that mat?"

Griffin picked up the thick sponge-rubber mat that the instrument case had been sitting on, and the two men started off down the tube, walking silently on the sponge-rubber-soled shoes which would not scratch the glass underfoot.

"Any indication yet as to who our saboteur is?" Griffin asked.

"I'm not sure," MacHeath admitted. "I've picked up a couple of leads, but I don't know if they mean anything or not."

"I wonder if there is a saboteur," Griffin said musingly. "Maybe it's just a run of bad luck. It could happen, you know. A statistical run of—"

"You don't believe that, any more than I do," MacHeath said.

"No. But I find it even harder to believe that a materialistic philosophy like Communism could evolve any workable psionic discipline."

"So do I," agreed MacHeath.

"But it can't be physical sabotage," Griffin argued. "There's not a trace of it—anywhere. It has to be psionic."

"Right," said MacHeath, grinning as he saw what was coming next.

"But we've already eliminated that. So?" Griffin nodded firmly as if in full agreement with himself. "So we follow the dictum of the Master: 'Eliminate the impossible; whatever is left, no matter how improbable, is the truth.' And, since there is absolutely nothing left, there is no truth. At the bottom, the whole thing is merely a matter of mental delusion."

"Sherlock Holmes would be proud of you, Bill," MacHeath said. "And so am I."

Griffin looked at MacHeath oddly. "I wish I was a halfway decent telepath, I'd like to know what's going on in your preconscious."

"You'd have to dig deeper than that, I'm afraid," MacHeath said ruefully. "As soon as my subconscious has solved the problem, I'll let you know."

"I've changed my mind," said Griffin cheerfully. "I don't envy your telepathy. I don't envy a guy who has to TP his own subconscious to find out what he's thinking."

MacHeath chuckled softly as he turned the bolt that opened the door in the "gun" end of the stripped-nuclei accelerator. The seals broke with a soft hiss. Evidently, the barometric pressure outside the two-mile-long underground tube had changed slightly during the time they had been down there.

"It'll be a week before we can test it," MacHeath said in a tired voice. "Even after we get it partly in balance. It'll take that long to evacuate the tube and sweep it clean."

It was the first sentence he had spoken in the past hour or so, and it was purely for the edification of the man who was standing on the other side of the air lock, although neither Griffin nor MacHeath had actually seen him as yet.

Griffin was not a telepath in the sense that the S.M.M.R. used the word, but to a non-psionicist, he would have appeared to be one. Membership in the "core" group of the Society for Mystical and Metaphysical Research required, above all, understanding. And, with that understanding, a conversation between two members need consist only of an occasional gesture and a key word now and then.

The word "understanding" needs emphasis. Without understanding of another human mind, no human mind can be completely effective. Without that understanding, no human being can be completely free.

And yet, the English word "understanding" is only an approximation to the actual process that must take place. Total understanding, in one sense, would require that a person actually become another person—that he be able to feel, completely and absolutely, every emotion, every thought, every bodily sensation, every twinge of memory, every judgment, every decision, and every sense of personal identity that is felt by the other person, no more and no less.

Such totality is, obviously, neither attainable nor desirable. The result would be a merger of identities, a total unification. And, as a consequence, a complete loss of one of the human beings involved.

Optimum "understanding" requires that a judgment be made, and that, in turn, requires two minds—not a fusion of identity. There must be one to judge and another to be judged, and each mind plays both roles.

Love thy neighbor as thyself. But the original Greek word would translate better as "respect and understand" than as the modern English "love." The founders of our modern religions were not fools; they simply did not have the tools at hand to formulate their knowledge properly. As understanding increases, a critical point is reached, which causes a qualitative change in the human mind.

First, self-understanding must come. The human mind operates through similarities, and the thing most similar to any human mind is itself. The next most similar thing is another human mind.

From that point on, all objects, processes, and patterns in the universe can be graded according to their similarity to each other, and, ultimately, to their similarity to the human mind.

Two given entities may seem utterly dissimilar, but they can always be linked by a tertium quid—a "third thing" which is similar to both. This third thing, be it a material object or a product of the human imagination, is called a symbol. Symbols are the bridges by which the human mind can reach and manipulate the universe in which it exists. With the proper symbols and the understanding to use them, the human mind is limited only by its own inherent structural restrictions.

One of the most active research projects of the S.M.M.R. was the construction of a more powerful symbology. Psionics had made tremendous strides in the previous four decades, but it was still in the alchemy stage. So far, symbols for various processes could only be worked out by cut-and-try, rule-of-thumb methods, using symbols already established, including languages and mathematics. None were completely satisfactory, but they worked fairly well within their narrow limits.

As far as communication was concerned, the hashed-together symbology used by the S.M.M.R. was better than any conceivable code. The understanding required to "break" the "code" was well beyond the critical point. Anyone who could break it was, ipso facto, a member of the S.M.M.R.

Most people didn't even realize that a conversation was taking place between two members, especially if a "cover conversation" was used at the same time.

MacHeath's verbal discussion of the testing of the nuclei accelerator was just such a cover. Even before he had cracked the air lock, he had known that Dr. Theodore Nordred was standing on the other side of the thick wall.

MacHeath pushed the heavy door open on its smooth hinges. "Oh, hello, Dr. Nordred.