Here comes the riddle. Suppose that your research is in gemology. Your special interest lies in the color properties of certain gemstones, in particular, emeralds. All emeralds you have examined before a certain time \(t\) were green (your notebook is full of evidence statements of the form “Emerald \(x\) found at place \(y\) date \(z (z \le t)\) is green”). It seems that, at \(t\), this supports the hypothesis that all emeralds are green (L3).