Is There Really One Fitrah?
Of course we can’t know with absolute certainty, however, armed with the knowledge of our own experiences as well as in the absence of any evidence that at some point in history humans used to like filth and dirt, we can conclude that our likeness for cleanliness is a-priori. In the real world there are no such things as proofs – we have to base our knowledge and actions upon inferences we draw from available evidences. The subjectivity of how clean is clean enough is an issue of application of our a-priori knowledge, and I agree that there can be differences on that amongst humans.