The Physician is a very learned man, having read all of the important medical authorities of his day. Not only that, but he's also something of an astrologer, relying upon the positions of the stars and planets, in addition to the more conventional theory of the humors, to find a cure for his patients. Whatever his methods, the Physician is skilled at finding the cause of his patients' illnesses, and once that's done, knows exactly where to send them for the cure: to his friend the apothecary, with whom he's worked out a financial deal that's made him rich. So wealthy is he that he's able to afford expensive blue and scarlet robes of taffeta and silk.
Yet, despite this one conspicuous display of wealth, the Physician is moderate overall, partaking of a diet that's modest and nourishing while saving up most of his gold. In a sly jab at the Physician, Chaucer tells us that he saves up his gold because "gold in phisik is a cordial" (General Prologue 445). It's true that medieval doctors and apothecaries used very finely ground gold in their most expensive potions, but what Chaucer probably means here is that gold is a "cordial," or pleasurable concoction, to the Physician because he loves money so much.
With the Physician, one of the most educated of the pilgrims, Chaucer provides us with an interesting contrast to the Clerk. While the Clerk's studies have been motivated by pure love of knowledge, to the detriment of the Clerk's financial situation, the Physician pursues his learning for financial gain. The comparison of these two highly-educated men allows us to weigh the consequences of both motivations for education.