The knowledge of diseases is the doctor's compass; the success of the cure depends on an exact knowledge of the disease'; the doctor's gaze is directed initially nut towards that concrete body, that visible whole, that positive plenitude that faces him the patient but towards intervals in nature, lacunae, distances, in which there appear, like negatives, 'the signs that differentiate one disease from another, the true from the false, the legitimate from the bastard, the malign from the benign'.