The relationship between a simple calculation (making no use of a distribution function) due to Butler and Buckingham, and previous work on relaxation processes (based on the Boltzmann or the Fokker-Planck equations) is examined. The Butler-Buckingham calculation is shown to correspond rigorously to a specific assumption as to the nature of the distribution function of the test particle: This assumption is shown to be valid for particles of high energy which relax by many small collisions. This approximation, in conjunction with its opposite extreme (the Mott-Smith approximation), covers essentially all relaxation problems.