If it is inconceivable that one could sincerely act on a maxim in a world where that maxim is a universal law of nature, then one is forbidden to act on that maxim.

Here is one way of seeing how this might work: If I conceive of a world in which everyone by nature must try to deceive people any time this will get them what they want, I am conceiving of a world in which no practice of giving one’s word could ever arise and, because this is a law of nature, we can assume that it is widely known that no such practice could exist. So I am conceiving of a world in which everyone knows that no practice of giving one’s word exists. My maxim, however, is to make a