success of jugglery and all the forms of tricks of sleight-of-hand; audiences fancy themselves to be seeing what they do not see. Casting our eyes upward to the sun and moon and stars, these heavenly objects seem to move with measurable slowness across the concave surface of the blue arch of sky; and only through the deductive reasonings and calculations of a Copernicus, a Galileo, a Newton, are we brought to the conviction that the earth is the moving object, that the blue vault but marks in the air the limitations of our vision, and that the shining stars that appear as candles in the sky are gigantic worlds moving with enormous velocity millions of miles away. Sitting in a railway-train at a station, as the train next to us on one side begins to move, we seem ourselves to be in motion, and only by looking on the opposite side and steadily observing some point or object that by previous observation we know to be fixed, can we correct our delusion; but in practical life we are not always able to find a fixed point or object external to ourselves by which we can distinguish the subjective and objective in our retinal impressions. Thus, in all human experience, "truth and lies are faced alike; their port, taste, and proceedings, are the same; we look upon them with the same eyes."[1]
Limitations of the Human Brain in Disease.—But the most serious blunders of the sense of sight, or indeed of the other senses, and indeed of reasoning in general, come from confounding the subjective with the objective. In certain states of the system, which are not rare but very common, and which may be either temporary or permanent, the brain has the power not only of modifying the impressions made by external objects over the retina, but of originating impressions even when there are no external objects corresponding to those impressions, and the individuals may have no way of distinguishing subjective from objective visions, or find it very difficult to do so without outside aid.
- ↑ A critic of Prof. Tyndall, indignant that the philosopher would not accept the reigning delusions of the day, declared that, when called upon to investigate any object, he would look at it, listen to it, touch it, taste it, and smell it, and then not believe it. The critic was not aware that, instead of censuring Prof. Tyndall, he was really giving one of the highest compliments that can be given to a scientific man.
This over-estimate of the capacity of the human brain and senses, united with the present chaotic state of the principles of evidence, affects injuriously not philosophy alone but practical life as well. In medicine, for example, it has for ages been the fashion to ignore or deride symptoms of a purely subjective nature, that have no corresponding lesions or morbid appearances that the aided or unaided senses can discover, and for the study of which it is necessary to depend on deductive reasoning and the statements of patients. This is in general the explanation of the fact that many of the most frequent and distressing diseases, such as nervous exhaustion, hypochondriasis, hysteria, hay-fever, and allied nervous affections, although of the highest scientific and practical interest, have, until quite recently, been almost entirely neglected, and the agonizing symptoms connected with them are dismissed as trifling if not imaginary. A broken leg every one can see, and touch, and handle; but an exhausted brain, oftentimes a far more serious matter, is passed by, and even its existence is doubted merely for this, that it is out of reach of the eye and the microscope.
