In this article, we comment on a primitive foresight of Galen's regarding the value of blood purification. His main arguments are based on: (i) The disease-blood concept, i.e., the idea that blood can be unclean (with an either excess or bad-quality humor) and thus cause a disease; (ii) Cure can be achieved if elimination (catharsis in Greek) of the humor (toxin) is possible; (iii) If the toxins are limited in the intravascular space, their elimination will be sufficient for cure by just a single attempt of replacing the unclean blood with pure; (iv) If the toxins have also affected the extravascular space, then repeated attempts will be needed; (v) The whole procedure can be compared with the insufficiency of washing once a dirty clay pot and immediately filling it with a pure liquid. The dirt that has adhered to the pot's walls will contaminate the pure liquid. Thus, repeated washing is needed to achieve cleanliness. Galen's metaphor of washing a dirty pot for a long period and eventually achieving a pure content is strikingly similar to the theory of hemodialysis. According to this, uremic toxins are spread both intravascularly and extravascularly. To eliminate them, long and repeated "washing" of the blood is needed. One of the reasons the first attempts to dialyze failed was that this "washing" procedure was inadequate (the other reasons were clotting of the blood, infections and access problems).