Some U.S. officials have said that the deal will, or must, let inspectors go “anywhere, anytime,” to make sure Iran isn’t enriching uranium in a covert facility. By contrast, the Ayatollah Khamenei has said he will not allow any deal that lets foreigners run free on Iran’s military bases. Is this an irreconcilable conflict? Maybe, maybe not.

Verification has always been a sore point in arms-control talks. The Soviet-American arms treaties, in Cold War times, never called for “on-site inspections,” because neither side wanted to give the other side such direct access to its sites. As everyone understands, there’s a fine line between inspections and espionage, especially when the signatories are still adversaries (and even when they’re not). The fact that Iran is allowing inspectors to roam its declared nuclear sites—that it’s allowing inspectors on Iranian soil at all—is remarkable, given the history between the two nations.

But what if inspectors suspect that some building is harboring a uranium-enrichment plant? How would a formal deal distinguish between inspections based on reasonable suspicions (derived from satellite imagery, communications intercepts, and so forth) and fishing expeditions for general intelligence? It’s a difficult issue; Khamenei’s concerns are hardly baseless or deceptive. There are ways to write such language into an international treaty or arrangement: procedures and protocols for requesting an inspection, empowering an international agency to rule on such requests. A key to this deal is to keep Iran from cheating—to make the deal palatable without having to rely on simple trust. The challenge is to draft language that both sides find acceptable and effective.