Page "Categorical imperative" Paragraph 42

Kant argued that any action taken against another person to which he or she could not possibly consent is a violation of perfect duty interpreted through the second formulation.

If a thief were to steal a book from an unknowing victim, it may have been that the victim would have agreed, had the thief simply asked.

However, no person can consent to theft, because the presence of consent would mean that the transfer was not a theft.

Because the victim could not have consented to the action, it could not be instituted as a universal law of nature, and theft contradicts perfect duty.