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.