Earlier 19th and 20th century scholars have similarly expressed a spectrum of views.
Winternitz,[REF] suggests that Isha Upanishad was probably a pre-Buddha composition along with Katha, Svetasvatara, Mundaka and Prasna Upanishad, but after the first phase of ancient Upanishads that were composed in prose such as Brihadaranyaka, Chandogya, Taittiriya, Aitareya, Kaushitaki and Kena.