The probability that a fair coin tossed yesterday landed heads is either 0 or 1,
but the probability that it would land heads was 0.5. In order to account
for the latter type of probabilities, past probabilities, a temporal restriction
operator is introduced and axiomatically characterized. It is used to construct
a representation of conditional past probabilities. The logic of past
probabilities turns out to be strictly weaker than the logic of standard
probabilities.