So neither the doctor's art nor any other art will create anything else out of health; for either nothing would be being produced, or else the opposite of health, if the same science were being employed (so too out of a house nothing could make its contrary): nor is there any other a which can make anything out of health, except as making a whole out of a part, as, for example, when the cobbler's art makes a shoe out of the front part of a shoe; for these two things can be produced out of one another by two processes, one of composition and the other of destruction.