Suppose a coin is tossed three times in succession, with results heads (\(H\)) or tails (\(T\)). In the theory of stochastic processes one constructs a sample space \(\SC\) containing 8 events, let us call them histories: \(HHH\), \(HHT\), \(HTH\), …. Notice that formally the same sample space is obtained if instead of a coin tossed three times in a row, three different coins are tossed simultaneously. Since in quantum mechanics the Hilbert space of several separate systems is constructed as a tensor product of the individual Hilbert spaces, this suggests that the Hilbert space for a system at a sequence of times, the histories Hilbert space, can be constructed as the tensor product of copies of the Hilbert space for a single time. We write it as