It's a matter of logical analysis: For a tooth to rot would take x units of time. Right? This is how the concept of decay works - in a particular extent of time, the decay takes place, until it is complete. For example, "It took uncle Tom's molar 45 years to rot." Now, if we keep the same conceptual structure, and replace the time unit "45 years" with "eternity", we get: "It took uncle Tom's molar an eternity to rot." When does eternity finish? Never. Ergo, the decay has never completed. Ergo, uncle Tom's molar never rot.