A well-known gap exists in Kant's argument that the categorical imperative is the fundamental principle of morality. The gap is between the claim that rational agents act only on maxims that could serve as universal laws and the more substantial requirement contained in the categorical imperative. There does not appear to be any obvious route from the seemingly trivial requirement to conform to universally valid laws to the controversial and substantive requirement of acting only on maxims that one can at the same time will that all other rational agents act on. Kant apparently assumed that we conform our wills to universally valid laws only if we act on maxims that we can will to be universal laws, but many have pointed out he gives no defence of this assumption. In what follows, I focus on a preliminary step to connecting the two claims, the step that connects rational agency to conformity to universally valid laws. Why must the will of a rational agent, insofar as he is rational, be such that it is universally valid in any sense? My hope is that by first considering this question, some light can be shed on the nature of the remaining argument for the categorical imperative.

Many readers assume that for Kant, a rational agent must conform to universally valid laws because rational agency is rational, and rationality requires this sort of universal validity.