Porphyry was Plotinus' pupil and biographer, and was known as a staunch opponent of Christianity. Nevertheless, he had an important impact on later Christian thought, particularly in logic (see, for instance, the Isagoge). His main contribution was to rethink the notion of the One that his teacher put forward, which was inherited from Parmenides through Plato. Porphyry did not regard Nous, or being, as a secondary level. He wrote of two different levels of being: to einai (the infinitive form), and ousia, the traditional substantive.The One becomes true existence (to einai).