given the initial condition p ( x , t = 0 ∣ ∣ x 0 ) = δ δ ( x − − x 0 ) ; where x ( t ) is the position of the particle at some given time, x 0 is the tagged particle's initial position, and D is the diffusion constant with the S.I. units m 2 s − − 1 (an indirect measure of the particle's speed). The bar in the argument of the instantaneous probability refers to the conditional probability. The diffusion equation states that the speed at which the probability for finding the particle at x ( t ) is position dependent.