The various schools of Hinduism, particularly those based on naturalism (Vaiśeṣika), rationalism (Samkhya) or ritualism (Mimamsa), questioned and debated what is Maya, and the need to understand Maya. The Vedanta and Yoga schools explained that complete realization of knowledge requires both the understanding of ignorance, doubts and errors, as well as the understanding of invisible principles, incorporeal and the eternal truths. In matters of Self-knowledge, stated Shankara in his commentary on Taittiriya Upanishad, one is faced with the question, "Who is it that is trying to know, and how does he attain Brahman?" It is absurd, states Shankara, to speak of one becoming himself; because "Thou Art That" already. Realizing and removing ignorance is a necessary step, and this can only come from understanding Maya and then looking beyond it.