Kamasutra 1.2.17

1.2.17

tiryag-yoniṣv api tu svayaṃ pravṛttatvāt kāmasya nityatvāc ca na śāstreṇa kṛtyam astīty ācāryāḥ.

Some learned men say that as Dharma is connected with things not belonging to this world, it is appropriately treated of in a book; and so also is Artha, because it is practiced only by the application of proper means, and a knowledge of those means can only be obtained by study and from books. But Kama being a thing which is practiced even by the brute creation, and which is to be found everywhere, does not want any work on the subject.