So, a big part of the Categorical Imperative is the idea that you should only do things that you would, at the time you do them, will to be universal (or natural law..). Kant wants you to test your motives for action to see if they pass this test — if they pass, your action is morally permissible.
Lots of students get this kind of wrong, mostly so they can object to it.. Here’s the real deal..
Kant wants you to think about the action you’re going to take, then ask yourself two important questions:
1) IF I do this thing (and it’s universal natural law), will I be able to complete the action?
2) Would I want others who are similarly situated to be able to do what I’m going to do?
On #1 — that’s the ‘would this be logically possible if everyone knew what my motive/intention is’ test. It would rule out lying (if telling a lie is universal, the person to whom you want to tell a lie will know you’re lying), it would permit doing things like visiting your grandma because you love her (everyone can visit their grandma, assuming she’s alive and worthy of love). It would permit things like expressing your talents — if you want to create art, everyone else can make art too… and would make it impossible for one person to use another as a means only (you’d know what they’re doing, thus if you participate you’re doing so with full knowledge of the situation.
On #2 — that’s the desirability test — there may be things that CAN be universalized, but the question is whether or not you want to live in a world like that? It would be logically possible to universalize an action like blondes always get to cut in line at Starbucks… This question asks if you would want to live in a world where people with blonde hair can always cut in line at Starbucks?
There is a lot more to Kant, but students often get this one wrong… they say that the theory implies that everyone should get to do brain surgery, whether or not they’re qualified — that’s not the point… you can’t universalize that either way (logically, would you want an unqualified brain surgeon cutting into your head, I wouldn’t… and I wouldn’t permit it). You also wouldn’t want to live in a world where the idiots who show how dumb they are every day on the internet are also permitted to do brain surgery without training.