[zmj]

You're not meant to retain your prior forever. When you update, you replace it. Think of it as a FOR loop over incoming evidence. On each iteration, you increment or decrement the probability of the proposition in proportion with the evidence.

Consider what the alternative would look like. You retain both your original prior and every piece of evidence you've ever seen, then recompute all of it each time you need the current value? No. That doesn't pass the intuition test either. What you now think of as your prior on the Universe is not actually the belief you held at birth. You've updated dozens or hundreds of times during childhood, and each time you discarded your old "prior".

Footnote: you do retain some of your recent evidence, for smoothing purposes. For more information, investigate how Bayesian networks incorporate time: http://en.wikipedia.org/wiki/Dynamic_Bayesian_network