In the kinetic theory of gases in physics, the molecular chaos hypothesis (also called Stosszahlansatz in the writings of Paul Ehrenfest) is the assumption that the velocities of colliding particles are uncorrelated, and independent of position. This means the probability that a pair of particles with given velocities will collide can be calculated by considering each particle separately and ignoring any correlation between the probability for finding one particle with velocity v and probability for finding another velocity v ' in a small region δr. James Clerk Maxwell introduced this approximation in 1867 although its origins can be traced back to his first work on the kinetic theory in 1860.