Today I share a long quote from Hermann von Helmholtz's Treatise on Physiological Optics. Helmholtz was one of the great intellectual figures of the 19th century, making seminal contributions in physiology and physics as well as psychology.
It might seem that nothing could be easier than to be conscious of one’s own sensations; and yet experience shows that for the discovery of subjective sensations some special talent is needed, such as PURKINJE manifested in the highest degree; or else it is the result of accident or of theoretical speculation. For instance, the phenomena of the blind spot were discovered by MARIOTTE from theoretical considerations. Similarly, in the domain of hearing, I discovered the existence of those combination tones which I have called summation tones.... It is only when subjective phenomena are so prominent as to interfere with the perception of things, that they attract everybody’s attention. Once the phenomena have been discovered, it is generally easier for others to perceive them also, provided the proper precautions are taken for observing them, and the attention is concentrated on them. In many cases, however – for example, in the phenomena of the blind spot, or in the separation of the overtones and combination tones from the fundamental tones of musical sounds, etc. – such an intense concentration of attention is required that, even with the help of convenient external appliances, many persons are unable to perform the experiments. Even the after-images of bright objects are not perceived by most persons at first except under particularly favorable external conditions. It takes much more practice to see the fainter kinds of after-images. A common experience, illustrative of this sort of thing, is for a person who has some ocular trouble that impairs his vision to become suddenly aware of the so-called mouches volantes in his visual field, although the causes of this phenomenon have been there in the vitreous humor all his life. Yet now he will be firmly persuaded that these corpuscles have developed as a result of his ocular ailment, although the truth simply is that, owing to his ailment, the patient has been paying more attention to visual phenomena. No doubt, also there are cases where one eye has gradually become blind, and yet the patient has continued to go about for an indefinite time without noticing it, until he happened one day to close the good eye without closing the other, and so noticed the blindness of that eye.
When a person’s attention is directed for the first time to the double images in binocular vision, he is usually greatly astonished to think that he had never noticed them before, especially when he reflect that the only objects he has ever seen single were those few that happened at the moment to be about as far from his eyes as the point of fixation. The great majority of objects, comprising all those that were farther or nearer than this point, were all seen double (Helmholtz 1856/1909/1962, vol. 3, p. 6-7, emphasis in original).
In a Cartesian (1641/1984) or Price-ian (1932) mood, it can seem almost impossible to doubt the correctness of your consequent judgments about your ongoing experience; but the leading figures of introspective psychology had quite the opposite opinion (as do I here and here). This was, no doubt, grounded in their experience of finding people disagreeing radically about their phenomenology, without any plausible physiological or behavioral or environmental differences underlying that disagreement; and of people changing their minds as their theories change, conforming too neatly to expectations, being swayed by the reports and opinions of their friends and advisors, and missing things that seem in retrospect to be obvious.
Consider Helmholtz’s own examples in this passage. The most familiar example to contemporary readers is the blind spot, which even in monocular vision can be very difficult to notice without aid. The musically or psychoacoustically trained will be familiar with combination tones and overtones, which are accompanying tones different in pitch from the fundamental tones produced by musical instruments. These tones surely add to our musical experience, but they can be very difficult to discern without training (see here for further explanation and a recreation the combination tone training exercises of Titchener 1901-1905). Whether Helmholtz is right about their being summation tones in particular (tones of the pitch characteristic of A+B, supposedly produced when sounds of frequency A and B occur together) remains unclear – buttressing his fundamental point. People sometimes notice bright afterimages – those that interfere with ordinary perception, especially, such as after having glanced at the sun – but rarely do they notice faint ones, which one might (with Helmholtz) think to be more or less a constant phenomenon of vision, or imperfections and floaters in the fluid that fills the eye, even when they’re looking for them; but is this imperfection in introspection, as Helmholtz supposes, or is our visual experience normally free of such perturbations?
Helmholtz’s final example is maybe the most striking: He suggests that most of the objects in the visual field, most of the time are seen double, but we fail to notice that. Reid (1764/1997) and Titchener (1910) and others make similar remarks (I discuss this also here and here). If you hold your finger near your nose and focus in the distance, the finger may seem to you to double. But is our visual experience of most objects like that? I can’t say it seems to me that way as I gaze about the room. But I haven’t had 10,000 trials of introspective training yet! Or maybe it’s Helmholtz and Reid and Titchener who are mistaken? But that only advances Helmholtz’s central point about the difficulty of the introspection. Or are we to suppose that Helmholtz and Reid saw most things double and the rest of us do not? – that everyone is right about his own experiences and wrong about everyone else’s? Besides the physiological and psychological implausibility of that (unless we see appropriate corresponding physiological or psychological differences), that supposition makes nonsense of people’s changing their minds....