5. "Only the physicians have a right to speak on this subject." Luzac says: "'Tis true that if the materiality of the soul was proved, the knowledge of her would be an object of natural philosophy, and we might with some appearance of reason reject all arguments to the contrary which are not drawn from that science. But if the soul is not material, the investigation of its nature does not belong to natural philosophy, but to those who search into the nature of its faculties, and are called metaphysicians." [65] 6. "Man is ... a machine." This is the first clear statement of this theory, which as the title of the work indicates, is the central doctrine of this work. Descartes had strongly denied the possibility of conceiving man as a machine. "We may easily conceive a machine to be so constructed that it emits vocables, and even that it emits some correspondent to the action upon it of external objects which cause a change in its organs,... but not that it should emit them variously so as appositely to reply to what is said in its presence, as men of the lowest grade of intellect can do." [66] 7. "Let us then take in our hands the staff of experience." La Mettrie repeatedly emphasizes the belief that knowledge must come from experience. Moreover he confines this experience to sense experience, and concludes "L'histoire naturelle de l'âme" with these words: "No senses, no ideas. The fewer senses there are: the fewer ideas. No sensations experienced, no ideas. These principles are the necessary consequence of all the observations and experiences that constitute the unassailable foundation of this work." This doctrine is opposed to the teaching of Descartes, who insists that "neither our imagination nor our senses can give us assurance of anything unless our understanding intervene." [67] Moreover Descartes believes that the senses are fallacious, and that the ideal method for philosophy is a method corresponding to that of mathematics. [68] Condillac and Holbach agree with La Mettrie's opinion. Thus, Condillac teaches that man is nothing more than what he has become by the use of his senses. [69] And Holbach says: "As soon as we take leave of experience, we fall into the chasm where our imagination leads us astray." [70] 8. "Galen (Galenus) Claudius, 130 to circa 210 A. D. An eminent Greek physician and philosopher. Born at Pergamus, Mysia, he studied both the Platonic and Peripatetic systems of philosophy. Satyrus instructed him in anatomy. He traveled extensively while young to perfect his education. About 165 A. D. he moved to Rome, and became very celebrated as a surgeon and practising physician, attending the family of Marcus Aurelius. He returned to Pergamus, but probably visited Rome three or four times afterwards. He wrote in philosophy, logic, and medicine. Many, probably most, of his works are lost. He was the one medical authority for thirteen centuries, and his services to logic and to philosophy were also great." [71]