[w] Compare Helmholtz on "The Sensation of Sight," Lectures, pp. 256, 7, and 259.
"We have already seen enough to answer the question whether it is possible to maintain the natural and innate conviction that the quality of our sensations, and especially our sensations of sight, give us a true impression of corresponding qualities in the outer world. It is clear that they do not. The question was really decided by Johannes Müller's deduction from well ascertained facts of the law of specific nervous energy. Whether the rays of the sun appear to us as colour, or as warmth, does not at all depend upon their own properties, but simply upon whether they excite the fibres of the optic nerve, or those of the skin. Pressure upon the eyeball, a feeble current of electricity passed through it, a narcotic drug carried to the retina by the blood, are capable of exciting the sensation of light just as well as the sunbeams. The most complete difference offered by our several sensations, that namely between those of sight, of hearing, of taste, of smell, and of touch—this deepest of all distinctions, so deep that it is impossible to draw any comparison of likeness, or unlikeness, between the sensations of colour and of musical tones—does not, as we now see, at all depend upon the nature of the external object, but solely upon the central connections of the nerves which are affected.... But not only uneducated persons, who are accustomed to trust blindly to their senses, even the educated, who know that their senses may be deceived, are inclined to demur to so complete a want of any closer correspondence in kind between actual objects and the sensations they produce than the law I have just expounded. For instance, natural philosophers long hesitated to admit the identity of the rays of light and of heat, and exhausted all possible means of escaping a conclusion which seemed to contradict the evidence of their senses.
"Another example is that of Goethe, as I have endeavoured to show elsewhere. He was led to contradict Newton's theory of colours, because he could not persuade himself that white, which appears to our sensation as the purest manifestation of the brightest light, could be composed of darker colours. It was Newton's discovery of the composition of light that was the first germ of the modern doctrine of the true functions of the senses; and in the writings of his contemporary, Locke, were correctly laid down the most important principles on which the right interpretation of sensible qualities depends. But, however clearly we may feel that here lies the difficulty for a large number of people, I have never found the opposite conviction of certainty derived from the senses so distinctly expressed that it is possible to lay hold of the point of error: and the reason seems to me to lie in the fact that beneath the popular notions on the subject lie other and more fundamentally erroneous conceptions."
[101] Is there, asks Idealistic Scepticism, any outside world at all?
We have all of us always believed in the veritable existence of this outside world from our childhood. So have we believed always in our own real and continued personal existence. The unyielding objectivities concerning which our senses inform us—the identical Self which receives their information—are entities no man ordinarily thinks of calling in question.
Let any one sit down and try to imagine himself a human animal let loose upon life without a firm belief in either of these two primary convictions. What could life be to him? to his descendants? to the world of men if similarly unbelieving? Yet what are the conditions or evidences of veracity upon which his and his fellows' present convictions must necessarily repose? Can he and others help believing them true? and why?—This "why" is a safe answer to the most plausible as well as the most refined objection against such primary beliefs as those premised by Natural Theology.
[102] Cheselden's case is reported in the Philosophical Transactions for 1728, and also in his Anatomy. Respecting the point above quoted he is confirmed by Mr. Nunneley, "On the Organs of Vision," 1858.
[103] See Dr. Carpenter's Principles of Human Physiology. Ed. 7. p. 713. § 635.
[x] The following quaint apology for our senses at the expense of our understanding may be new to the majority of my readers:—
"We have seen two notorious instances of sensitive deception, which justifie the charge of Petron. Arbiter.