It is of primary importance to determine the correct signification of the expression _consciousness_.[38] There are few words in philosophical language which have such diverse acceptations. As my conception of consciousness is somewhat special without at the same time being peculiar to me, I owe it to my readers to say what I mean to designate by this term. [38] The French have but one word to express what is meant in English by the word _Conscience_ (_i.e._ the principle which decides on the lawfulness or unlawfulness of our actions or desires), and the word _Consciousness_ (_i.e._ the being aware, the knowing of one’s own thoughts). Nevertheless we consider this chapter could ill spare this masterly synthesis.—_Note of Translator._ I conceive consciousness, _lato sensu_, as a function of living matter. It is the particular state which determines in organised and living matter another state of the centre where this matter lives. It is, if you like, a kind of reaction of the living matter in harmony with external phenomena. This mode of reaction, like every other mode of reaction, allows of two conditions: some sort of sensibility to the action of the ambient, permitting variations thereof to be felt; some sort of activity which permits of realising an adaptation to the ambient, and of producing internal modifications corresponding, in some measure, to the perceived external modifications. In order that the internal modifications may realise this equilibrium, they must not go beyond a certain amplitude, whence the theoretic necessity for the _sensibility_ to be always apprised of the internal modifications of the living substance, as it perceives the external modifications of the ambient. Experience proves that in reality things do happen in this way. In fact, we are able in the animal kingdom to prove the existence of special organs, some of them destined to the perception of the successive states of the ambient and of the individual, the others to the active realisation of the latter to the former. The different modifications provoked in the _receptive_ system by the variations of the ambient, determine in their turn the intervention of the _active_ system which realises the internal variations. This is the principle of the nervous and muscular systems, the latter being only put into play by the former; natural history shows us the progressive specialisation of these nervous and muscular elements. At first non-differentiated in appearance, the animal cell presents in more complicated animals a sensitive pole and an active pole, the one nervous, the other muscular. The myo-epithelial or neuro-muscular cells offer us a classical example in the hydra.