Universalizability reductio ad absurdum

Question

My 13-year-old son is attending philosophy camp and one of his homework assignments is to come up with some objections to Kantianism. In a previous conversations I suggested a ridiculous counter-example for Kant's universalizability principle ("Act only according to that maxim whereby you can at the same time will that it should become a universal law.") As my son generously appreciates my jokes more than anyone else does, it stuck with him and he's thinking of using it. I don't actually understand the universalizability principle well enough to defend it from my joke, though I can't imagine there wouldn't be a pretty simple argument against it. I found this, but I was hoping for something more concise. Here is the example:

• If I think it's ok for me to pick my nose, then I should consider it permissible for everybody to pick my nose.

Can anyone give me a one or two sentence defense against this example?

Edit. Rephrasing as a maxim and contradiction in response to @ChristopherE's answer and comments:

• When I'm bored, the first thing I should do to pass the time is pick my nose, which would be hard to do in a world in which everyone picked my nose when they were bored.

(The whole question, of course, is reminiscent of the classic joke maxim: "You can pick your friends and you can pick your nose, but you can't pick your friend's nose," as well as a variation on it I heard a couple decades ago: "You can pick your friends and you can pick your nose, but you can't legislate morality.")

Answer 1

One aspect of the universalization test these examples overlook is its mention of “maxim.” A maxim is a rule of action, implicitly laid down by an actor in the actor's will. One way to characterize maxims is that they specify circumstances, goals, and actions. An example is: “When I see someone in need, then in order to be the best person I can be, I will help then.” In those terms, neither "It's okay to pick my nose," nor "I think it's okay to pick my nose" are fully specified maxims.

The reason to focus on the structure of maxims here is that all the components are often required to expose the contradiction in the will that is supposed to result from universalization of (at least some) wrong actions. In terms of Kant's most famous example, the idea that everyone makes lying promises doesn't actually create a contradiction in the will. There is a possible world in which everyone makes lying promises. The contradiction arises in imagining the possible world in which everyone makes lying promises when they are in need, in order to improve their situations. The contradiction now exposed is that the lying promises can't be expected work to address one's need in the possible world where everyone is always lying.

I see you asked for a “one or two sentence defense.” Okay! “Those aren't maxims, and the universalization test only applies to maxims.”

There is another problem here, by the way. You haven't correctly universalized the statements. Universalized forms of “I think it's ok for me to pick my nose” include “Everyone thinks it's okay for me to pick my nose,” and “Everyone thinks it's okay for themselves to pick their noses,” but not “I think it's okay for everyone to pick my nose.” The last case universalizes a predicate inside the sentence (“pick”), not the whole sentence.