Consider a classical particle – the position of which is not known with certainty. We must deal with it statistically, so only the probability density ρ ( x , t ) {\displaystyle \rho (x,t)} is known. Probability must be conserved, i.e. ∫ ρ d 3 x = 1 {\displaystyle \int \rho \,d^{3}x=1} for each t {\displaystyle t} .