Galen believed in the doctrine of humours originated by Hippocrates, which supposes the condition of the body to depend upon the proper mixture of the four elements, hot, cold, moist and dry, and that drugs possess the same elementary qualities, and that on the principle of contraries one or other was indicated, e.g.
In drugs were to be recognized the same elementary qualities - hot, cold, moist, dry, &c. - as in the human body; and, on the principle of curing by contraries, the use of one or other was indicated.
For admitting that contraries co-exist for the perceiving subject, he was able to assert the co-existence of contrary qualities in the same object.
The doctrine of the unity of contraries so far from being the denial of the law of non-contradiction is founded on an absolute reliance upon it.
Monsignor Barnes is rather too apt to employ the method of interpretation by contraries, on the ground that in such letters the writer always concealed the real facts.