While his works in medicine indicate an in-depth theoretical knowledge in medicine of his time, he likely had limited expertise as a practitioner, and declared in one of his works that he had not "practiced much apart from myself, my relatives or my friends." He did serve as a royal physician, but his qualification and education was mostly theoretical. For the most part, Averroes's medical work Al-Kulliyat fi al-Tibb follows the medical doctrine of Galen, an influential Greek physician and author from the second century, which was based on the four humors —blood, yellow bile, black bile, and phlegm, whose balance is necessary for the health of the human body. Averroes's original contributions include his observations on the retina: he might have been the first to recognize that retina was the part of the eye responsible for sensing light, rather than the lens as was commonly thought. Modern scholars dispute whether this is what he meant it his Kulliyat, but Averroes also stated a similar observation in his commentary to Aristotle's Sense and Sensibilia : "the innermost of the coats of the eye [the retina] must necessarily receive the light from the humors of the eye [the lens], just like the humors receive the light from air."