2. Several German authors have maintained in recent years, that if the nervous system intervenes in the perception of external objects, it is a faithful intermediary which should not work any change on those physical actions which it gathers from outside to transmit to our consciousness. From this, point of view colour would exist as colour, outside our eyes, sound would exist as sound, and in a general way there would not be, in matter, any mysterious property left, since we should perceive matter as it is. This is a very unexpected interpretation, by which men of science have come to acknowledge the correctness of the common belief: they rehabilitate an opinion which philosophers have till now turned to ridicule, under the name of naive realism. All which proves that the naivete of some may be the excessive refinement of others. To establish scientifically this opinion they batter down the theory of the specific energy of the nerves. I have recalled in a previous page[10] of what this theory consists. I have shown that if, by mechanical or electrical means, our different sensory nerves are excited, notwithstanding the identity of the excitant, a different sensation is provoked in each case--light when the optic nerve is stimulated, sound when the acoustic, and so on. It is now answered to this argument based on fact that the nature of these excitants must be complex. It is not impossible, it is thought, that the electric force contains within itself both luminous and sonorous actions; it is not impossible that a mechanical excitement should change the electric state of the nerve affected, and that, consequently, these subsidiary effects explain how one and the same agent may, according to the nerves employed, produce different effects. 3. After the spiritualists and the experimentalists, let us take the metaphysicians. Among them one has always met with the most varying specimens of opinions and with arguments for and against all possible theories.