Supposing we put a photographic plate into the camera and expose it, then take it to the dark room and look at it. Can we see any change? None whatever. But immerse it in the developing solution, and the change it has undergone becomes apparent,--there is a picture on our plate. If we take it out into the light without first fixing it, the picture fades away. Is the plate therefore the same as it was before we took our picture? No, it is entirely different, and nothing in the world can ever restore the film on the plate precisely to its former condition. In like manner outer impressions are every moment altering our nervous structure, and life’s clock cannot be put back. CHAPTER V. _OBSERVATION._ We must bear in mind the fact that all observation is difficult, not only because of our lack of perception, but because impressions already received project themselves, so to speak, on to the objects to be observed, and prevent our seeing these as they really are. Here lies the true interpretation of the “fixed idea,” and the “subjective” order of mind. The mind must not only be large enough and elastic enough to receive new impressions, but must also combine them with the old, so as to modify the former idea; for that which can only receive a new impression at the cost of casting out former and equally truthful impressions is deficient in retentiveness, and must always be wanting in thoroughness. We see examples of this order of mind in persons who eagerly take up one subject after another, but who permanently assimilate nothing. Mental growth is manifestly of little advantage to us if, while gaining on the one side, we are continually losing on the other. Again, if we cut ourselves off from outer impressions we lose mental power, for it is only by use of a part that nutriment can be attracted to it; and if our impressions are too exclusively of one particular order, our minds become ill-balanced. Before we can be of special use as observers, therefore, education must have meant for us “the harmonious development of all the faculties.” We need to have our sense perceptions in good order that we may perceive quickly and accurately; we need the retentive power to enable us to store the impressions received; and we need the faculty of combining these impressions in our minds--the lower manifestation of which process we call imagination, and the higher, reasoning faculty.