But the best provings of the pure effects of simple medicines in altering the human health, and of the artificial diseases and symptoms they are capable of developing in the healthy individual, are those which the healthy, unprejudiced and sensitive physician institutes on himself with all the caution and care here enjoined. He knows with the greatest certainty the things he has experienced in his own person.1
1 Those trials made by the physician on himself have for him other and inestimable advantages. In the first place, the great truth that the medicinal virtue of all drugs, whereon depends their curative power, lies in the changes of health he has himself undergone from the medicines he has proved, and the morbid states he has himself experienced from them, becomes for him an incontrovertible fact. Again by such noteworthy observations on himself he will be brought to understand his own sensations, his mode of thinking and his disposition (the foundation of all true wisdom), and he will be also trained to be, what every physician ought to be, a good observer. All our observations on others are not nearly so interesting as those made on ourselves. The observer of others must always dread lest the experimenter did not feel exactly what he said, or lest he did not describe his sensations with the most appropriate expressions. He must always remain in doubt whether he has not been deceived, at least to some extent. These obstacles to the knowledge of the truth, which can never be thoroughly surmounted in our investigations of the artificial morbid symptoms that occur in others from the ingestion of medicines, cease entirely when we make the trials on ourselves. He who makes these trials on himself knows for certain what he has felt, and each trial is a new inducement for him to investigate the powers of other medicines. He thus becomes more and more practised in the art of observing, of such importance to the physician, by continuing to observe himself, the one on whom he can most rely and who will never deceive him; and this he will do all the more zealously as these experiments on himself promise to give him a reliable knowledge of the true value and significance of the instruments of cure that are still to a great degree unknown to our art. Let it not be imagined that such slight indispositions caused by taking medicines for the purpose of proving them can be in the main injurious to the health. Experience shows on the contrary, that the organism of the prover becomes, by these frequent attacks on his health, all the more expert in repelling all external influences inimical to his frame and all artificial and natural morbific noxious agents, and becomes more hardened to resist everything of an injurious character, by means of these moderate experiments on his own person with medicines. His health becomes more unalterable; he becomes more robust, as all experience shows.