CERTAIN considerations based on the principles of quantum mechanics lead to the view that the process of localization of a particle in space and time is subject to quantization. Thus if the world line of a particle of rest mass m0 is considered, it has no physical meaning to subdivide it into elements of length less than h/m0c, or, what is the same thing, to associate with the particle intervals of proper time less than h/m0c2 1. This has been expressed as a principle of elementary indeterminacy by Ruark2 and by Furth3.