Organon §107

A medicine's actions cannot be distinctly perceived in a sick person because the the medicinal alterations are confounded with the symptoms of disease.

If, in order to perform this investigation, medicines are only given to sick persons (even when only one simple and singly prescribed medicine is given) one sees little or nothing definite of their pure actions since the particular condition-alterations expected from the medicines are mingled with the symptoms of the present natural disease, and are therefore seldom distinctly perceptible.