At this point SDI came into the negotiations. Gorbachev would consider such a move only if the US limited their SDI efforts to the laboratory for ten years. Excalibur, which Teller's letter of only a few days earlier once again said was ready to enter engineering, would need to be tested in space before that point. Reagan refused to back down on this issue, as did Gorbachev. Reagan attempted one last time to break the logjam, asking if he would really "turn down a historic opportunity because of a single word" ("laboratory"). Gorbachev said it was a matter of principle; if the US continued real-world testing while the Soviets agreed to dismantle their weapons, he would return to Moscow to be considered a fool.