Born put Heisenberg's equation into a matrix form. Born immediately noticed that to calculate using these matrices, one matrix had to be multiplied by another matrix. In matrix multiplication, multiplying matrix P by matrix X gives a different result from the one obtained by multiplying matrix X by matrix P. He further noticed that the difference between the two matrices involves h, the Planck proportionality constant. It shows that one can never pin down things like P (momentum) and X (location) exactly. As a result, we cannot pin down a time any better than we can pin down a place or a momentum. We get closer and closer and then we reach the "Planck time."