Sen. William S. Cohen (R-Maine), a poet by avocation, observed the other day that spying is a "wilderness of mirrors," a business built on "many layers and levels of deceit."

Therefore, he concluded, it is probable that we will never find the consolation that "truth" might bring to the strange affair of Vitaly Yurchenko, a defector from the Soviet KGB who returned to the motherland yesterday, loudly proclaiming that he had been tortured by a crazed Vietnam veteran named "Charlie" during three months in custody of the Central Intelligence Agency.

One piece of the elusive "truth" Cohen and his colleagues on the Senate Select Committee on Intelligence would choose to possess is whether the Yurchenko episode had any large meaning at all. Did he bring to the CIA -- or take from it -- anything of value? Or is the incident no more than another testament to the Andy Warhol theory that in the contemporary world everyone is destined to be famous for 15 minutes?

Early on, according to Sen. Malcom Wallop (R-Wyo.), the CIA informed the Senate committee that Yurchenko was "a big fish . . . a juicy morsel." The State Department in October described him as the deputy director of the KGB's North American spying apparatus. Others referred to him as the "No. 5 man" in the KGB hierarchy, the "most important KGB defector" of all time.

The adjectives are provocative but not illuminating. The central question is: What did this "big fish," this "deputy director," do in the intelligence bureaucracy of the Soviet Union? What did he know?

At a news conference Monday, Yurchenko called himself a "security officer" but gave no job description. On Wednesday, anonymous CIA officers downgraded his importance. He had a rank comparable to colonel, they said, functioned primarily as an administrator who handled paper but not spies, and may not have had any active role in the management of Soviet espionage networks in the United States. President Reagan said the information he provided the CIA was nothing "new or sensational." Others described his revelations as "chickenfeed."

That is the trouble with the world of "deceit" Cohen described, especially in the twilight wars intelligence agencies wage against each other. Deceptions are so much a part of the game that both the players and their confessions are forever tainted.

U.S. intelligence analysts, more than 20 years after the event, are still debating whether Yuri Nosenko -- who left the Soviet Union soon after the assassination of John F. Kennedy with the news that Lee Harvey Oswald had no connection to any official Soviet agency despite spending a year in Russia -- was a genuine defector or a "mole."

Yurchenko, in theory, could have delivered to the CIA and the FBI the names of Soviet agents and informers in the United States and Canada. He could have, in theory, shed light on the types of information the KGB covets on this continent and the methods it uses to acquire it. This could have eased the U.S. counterintelligence burden, saved the government a great deal of time and money and forced the Soviet Union into the painful process of building new espionage networks.

He might have possessed, in other words, valuable intramural information: spy secrets. There has been no suggestion, however, that he knew much of anything else. He would have had no blueprints for a new weapons system, no minutes of Politburo meetings, no details of arms negotiating positions.

How much information Yurchenko provided is something not yet in the public domain. "Chickenfeed," his detractors are saying with CIA encouragement. But that in itself could be another deceptive ploy in the intelligence game.

The harm caused to U.S. interests by the Yurchenko affair is as difficult to assess as Yurchenko himself. It left the CIA, as one of its officials said, with egg on its face, "probably the whole carton." In other words, it was a public relations fiasco. There are other concerns in the Senate.

Wallop is worried about what Yurchenko might have learned from his handlers. He is worried that Yurchenko may have "guided our presummit thinking . . . by diverting and misinforming" the CIA. He is worried about weaknesses in the agency's counterintelligence section and about the CIA's tolerant attitude, expressed, he said, by a CIA official who thought "searching for a mole is more disruptive to the agency than the mole."

Cohen is fearful that the Yurchenko case and the publicity surrounding it may discourage other defectors from dealing with an "inept" agency: "If he was a plant, we were inept in dealing with him. If he was a real defector, we were inept in dealing with him." And that, among other things, is damaging to the agency's morale.

These intangible wounds may constitute the biggest harm from the Yurchenko case. No one argues that it was a "disaster" or "catastrophe." No heads are likely to roll, in Cohen's view, and CIA Director William J. Casey will suffer no "long-term damage." The spy and counterspy bureaucracies will get on with their trade in the "wilderness of mirrors" and perhaps encounter Vitaly Yurchenko again some day.