  Prices of tradables can only be expressed relative to each other at any
instant of time. This fundamental fact should therefore also hold for contigent
claims, i.e. tradable instruments, whose prices depend on the prices of other
tradables. We show that this property induces local scale-invariance in the
problem of pricing contingent claims. Due to this symmetry we do not require
any martingale techniques to arrive at the price of a claim. If the tradables
are driven by Brownian motion, we find, in a natural way, that this price
satisfies a PDE. Both posses a manifest gauge-invariance. A unique solution can
only be given when we impose restrictions on the drifts of volatilities of the
tradables, i.e. the underlying market structure. We give some examples of the
application of this PDE to the pricing of claims. In the Black-Scholes world we
show the equivalence of our formulation with the standard approach. It is
stressed that the formulation in terms of tradables leads to a significant
conceptual simplification of the pricing-problem.
