Kant's categorical imperitive

[Goethe]

It's immoral not to be a farmer since, if no one was a farmer, we'd all starve

What is your motive for staying out of the farming profession? And the motive for that? And so on?

Keep doing this until you find the metaphysical assumption (we've all got a set of those) on which it is based.

Now ask, "What would it be like if everyone lived by this assumption?"
Does it create a contradiction?
If so, the CI says you should drop that assumption. And if the answer to the first question is distasteful to you - that also means you should drop the assumption.

Maybe not precise but it gives you the gist of the CI.