What is "holiness"?

It is difficult to give a clear answer here. Simply put, holiness is a law of the haftza, and this means that there is something in the thing itself. Our attitude towards it stems from its holiness, but it is not the essence of holiness. Although many have argued that the second option is the correct one. This is related to Yochanan Silman's articles on nominalism and realism of halacha, and in particular there is an article by Haneska on the Maimonides' method on the question of impurity and holiness. I remember that he claimed that these are laws of the gabra and not of the haftza, but I think he is wrong (I remember that I was not convinced by his views either). Leibowitz always claimed that this was a law of the gods, and his view is from the Mishnah of Kelim, chapter 1, for example, on the holiness of the Land of Israel: "And what is its holiness? That an omer and two loaves of bread are brought from it." Although this should be lightly rejected, since the Mishnah there only brings halachic implications (regarding all ten holy things) but does not necessarily define holiness in this way per se.