Given an arbitrary initial value x/sub 0//sup -/ for the differential-algebraic equation Ax/spl dot/(t)+Bx(t)=f(t), an initial value x/sub 0//sup +/ can be selected from among all consistent initial values for that equation by means of the Laplace transform. We show that this choice is the only one that fulfils some simple, physically reasonable assumptions on linear systems' behavior, thereby ruling out other values of x/sub 0//sup +/ proposed in the literature. Our derivation is elementary compared to previous justifications of the above Laplace transform based method: We also characterize x/sub 0//sup +/ by means of a system of linear equations involving A, B, derivatives of f, and x/sub 0//sup -/, which gives a new method to numerically calculate x/sub 0//sup +/.