Answer: Act always on that maxim which you can, at the same time, will should be universal law."
Explanation: Kant was searching for that essential grounding for ethics that would account for the morality of acts, define what was a moral act, apply to all moral acts, and apply to all rational beings. The Categorical Imperative is categorical because it is intended to be universal - applies to all moral agents in all moral situations; it is an imperative because the act is done only for the sake of acting in a moral way - it is like a command, not a piece of advice.