The recent popularity of sex reassignment surgery is logically untenable and immoral when understood in the light of Kantian philosophy. From a Kantian perspective of synthetic a priori judgments, I argue that a biological male cannot rationally claim to “feel like a woman inside.” As a male, any female is part of the noumenal world and cannot be known apart from perception. The statement “I feel like a woman inside” assumes all women feel the same on the inside. Kant’s explanation of the noumenal and phenomenal excludes the possibility of knowing that all women or men feel the same inside because it is impossible to know the noumenal world or the thing in itself. I argue that Kant’s moral principle of universality would conclude sex reassignment surgery would lead to contradictions and absurdities. If all men everywhere universally had the operation to look like their perception of a female, the end of the human race would soon follow; therefore, a Kantian would conclude sexual reassignment surgery is immoral.