Severus, thankfully, took pity on her confusion and kept explaining. "If there is one thing that we can count on Death Eaters to want desperately, it is the Philosopher's Stone," he said. "After all, they're all fundamentally self-absorbed, and if ever there was an easier ticket to a selfishly comfortable life than endless gold and immortality, I cannot name it." Minerva noted with interest and approval that Severus was now referring to the Death Eaters as they, rather than we. He continued, "We can safely expect that any who find out will attempt to steal it." Reasonable enough. She nodded. "Therefore, a theft gives us useful information about who is and is not trustworthy. For example, if Lucius Malfoy were the thief, one might safely assume that I was the one who had told him."