Of late years, science has analysed more closely, than beforetime was practicable, the relations connecting the organic with the inorganic world. It has demonstrated that the substance of animated beings does not differ from that of inert and insensible bodies; life lays hold of its materials in the physical world without thereby altering their fundamental properties, and death returns them un-impaired to that abyss of material substance, whence they have been for a moment abstracted to be clothed in ephemeral forms. Science has made a further step; she is not content to prove the true and lasting identity of the simple bodies diffused through the inorganic and the organic kingdom; by a re-union of all the parts she has formed, if not a living being, at least the constituent parts of organisms; she has not formed a flower, a fruit, or a muscle, but she has made the chemical principles from which they are extracted. Will she go yet further ? Will she be able some day to control or order those mysterious forces which unite these principles, so as to make of them true organisms, and combine these organisms together, and cause them to co-operate in the formation of a common and individual action? We may doubt this, and it even needs some audacity to propound such a question. Such problems can only present themselves to us within that, vague and uncertain limit which separates the domain of science from that of metaphysics. Happy are they who, limiting their desires and their hopes, are content to extract some secrets from the world of phenomena, patiently analysing its laws and registering its facts, without seeking to penetrate the very essence of natural forces, or of the substance which they put in movement! He, on the contrary, who takes in the whole world in his ambitious investigations, who will not accept the convenient duality of mind and matter, who wishes at least to reconcile their limits, and to fix their point of contact, condemns himself to strange doubts, which scientific certainty cannot yet completely dissipate. Yet are there certain minds who cannot resist the attraction of these problems. Science always leads us towards man, and man towards philosophy. All optical science is in our eye—all acoustic science in our ear. The weight which old age drags, and which youth carries with so facile a grace, is that which binds the worlds in their orbits. The caloric which animates our bodies is a portion of the universal heat; the nerves are telegraphs which imprint upon the brain the sensation produced by our surroundings, and which transmit the dictates of our will to the senses. All the forces of nature, without exception, have been put in requisition to create the wondrous composition called man. Time, space, the world at large, can teach us nothing that we cannot study in him, and in him we shall find more than we can ever discover elsewhere. Man is not only a weight, a combination of chemical atoms, an aggregate of the most delicate physical instruments, he is besides all this, a personal force. It is not, therefore, without reason that biology or the study of life has been the keystone, of the scientific edifice. After traversing the numerous circles of human knowledge, we are forcibly led to this centre, which on one side metaphysics takes as its point of departure, only studying the being in itself, without form or exterior support, without definite action upon that which surrounds it; on the other hand, science considers it chiefly in its manifestations, and only approaches by degrees that unknown which lies under the phenomena. These two methods each present a legitimate operation of the mind. To proceed from object to subject, or inversely from subject to object, is it not to clear the same interval, to pass over the same abyss? To study the relations of the corporeal substance with the hidden substance which regulates its movements, such is the grand problem of metaphysics; such also is the final aim of science. The former has more immediate reference to the mind, the latter to the life; but we do not know more of the mind than we do of the life, and under these differing terms is doubtless hidden one and the self-same mystery. Is the principle of life different from that of the mind ? or, on the contrary, is it identical? Who are right, the materialists who identify mind and matter; the vitalists, who interpose life as a bond between body and mind; the animists, who make the mind the source and the principle, not only of intellectual phenomena, but also of organic functions? Such are the grave questions which I desire to examine with the aid of the most recent labours of metaphysicians and men of science. The physiological school of Montpellier did not invent vitality, neither did Stahl discover animism; the germs of these great doctrines are to be traced in the remotest antiquity. In reality, one can scarcely apprehend that man exists without demanding from himself, in terms more or less precise, what relation he bears to the rest of the world, in what manner he differs from inert matter, if the secret principle which gives him life and thought must die with him or survive him. But these formidable questions do not assume the same aspect in the minds of all, and even in the same individual there are moments when they are rejected as useless, and other moments when they intrude themselves with irresistable authority. That which is true of man is true also of humanity. One of the strongest attractions of the history of philosophy is to show the successive weaknesses, the victories, and transports of a great soul which develops itself in time and circumstance. A rapid glance at the modern treatises will show what numerous solutions the problem of the mind has already received on the part of the materialists, the animists, and the vitalists, in France and Germany; we shall endeavour to show in what the several schools differ, and what sources of enlightenment must be sought from natural as well as from historical science.