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Besides his decisive impact on phenomenology, Kant also deeply influenced the development of scientific psychology, albeit in a more indirect way. While the phenomenologists pursued an openly Kantian project at the margins of the mainstream neo-Kantian philosophy of the late nineteenth century, the rise of scientific psychology as an academic discipline separate from philosophy took a more tortuous path. For Kant argued against the very possibility of scientific psychology, both in his Critique of Pure Reason and in his Metaphysical Foundations of Natural Sciences (MFNS). Given Kant’s stature in the German academy of the nineteenth century, his arguments needed to be addressed by anyone who wished to engage in scientific psychology.

Thus Wilhelm Wundt, the founder of the world’s first experimental psychology lab in 1879 at Leipzig and widely known as the “founding father of psychology,” addresses Kant’s arguments in the early pages of his textbook Principles of Physiological Psychology (1874/1902/1904). He begins with a summary of Kant’s arguments from MFNS:

Kant once declared that psychology was incapable of ever raising itself to the rank of an exact natural science. The reasons that he gives for this opinion have often been repeated in later times. In the first place, Kant says, psychology cannot become an exact science because mathematics is inapplicable to the phenomenon of the internal sense; the pure internal perception, in which mental phenomena must be constructed – time – has but one dimension. In the second place, however, it cannot even become an experimental science, because in it the manifold of internal observation cannot be arbitrarily varied – still less, another thinking subject be submitted to one’s experiments, conformably to the end in view; moreover, the very fact of observation means alteration of the observed object. The first of these objections is erroneous; the second is, at the least, one-sided. (1874, p. 6)

Wundt responds to these objections to the possibility of scientific psychology by drawing on nineteenth-century advances in sensory physiology.

For most of the nineteenth century, our current understanding of the division between the sciences and humanities was not yet in place. Hermann Helmholtz, for example, made crucial contributions in geometry, physics, physiology, and philosophy. In the 1840s, Helmholtz was a member of the Berlin Physical Society, whose members signed an oath to explain life and consciousness in terms of only known physical and chemical principles. Consciousness, that is, is made of the same stuff, obeying the same laws, as everything else in the universe. Helmholtz invented the ophthalmoscope (1850), making it possible to study vision in greater detail than ever before. He also was the first to measure the velocity of neural conduction, which he estimated as 25–40 meters per second. Combined with his views that perceptual experience was nothing other than the activity of neurons, this implies that our experience is not of what is happening right now, but of the very recent past. These and many other findings lead Helmholtz to a detailed theory of perception as unconscious inference that is equally physiological and philosophical and that continues to inform twenty-first-century views of perception.

Along with Helmholtz’s work, members of the Berlin Physical Society and their fellow travelers produced innovations in psychophysics, seeking correlations between changes to the material world and changes in the mind that experiences it. Gustav Fechner’s Elemente der Psychophysik (1860; Elements of Psychophysics) describes decades of work by Ernst Weber and Fechner himself, among others, from the middle 1800s. Research in this period by Weber, for example, focused on just noticeable differences (jnd), differences in physical magnitudes (mass, brightness, intensity of sound) that are sufficient for an experimental participant to notice. For example, participants are given a standard object and asked to wield test objects each of which is slightly heavier (or lighter) than the standard object and to report when they get to a heavier (or lighter) one. The jnd is the mass difference (measured by a non-human device) that leads to a difference in human-experienced mass. Studying this in multiple sensory modalities led to what is now known as Weber’s law: the ratio of the change that leads to a jnd to the standard object is a constant. So, if I is the magnitude of the standard object and DI is the size of the jnd,

For example, suppose that for a 20-gram standard mass, participants do not notice that an object is heavier until it weighs at least 22 grams and do not notice that it is lighter until it weighs less than 18 grams (these numbers are for illustration only). Then DI = 2, and DI/I = 2/20 = 0.1. According to Weber’s law, the jnd for a 60-gram standard object should be 60 grams * 0.1 = 6 grams, so participants should not notice a difference in weight between the standard object and a test object until the test object weighs 54 grams or less, or 66 grams or more. Similarly, there should be no noticeable differences between masses between 90 and 110 grams for a 100-gram standard object (100 grams * 0.1 = 10 grams), or between masses of 900 and 1,100 grams if the standard object weighs one kilogram (1,000 grams * 0.1 = 100 grams), and so on.

These findings provide Wundt with a reply to Kant’s first objection, that mathematical methods cannot be applied to perception. Wundt points out that time is not the only variable in experience: there is also the quality of the sensation, which is proportional to the stimulus. “But, as a matter of fact, our sensations and feelings are intensive magnitudes, which form temporal series. The course of mental events has, therefore, at any rate two dimensions; and with this fact is given the general possibility of its presentation in mathematical form” (1874, p. 6).

This takes care of Kant’s first objection to scientific psychology. Wundt also relies on psychophysics to rebut Kant’s claim that one cannot get knowledge of the mind from introspection. First, Wundt agrees with Kant that ordinary introspection is unreliable. Psychophysics, though, shows that one can nonetheless measure experience. It is worth quoting Wundt at length on this:

The arguments that Kant adduces in support of his second objection, that the inner experience is inaccessible to experimental investigation, are all derived from purely internal sources, from the subjective flow of processes; and there, of course, we cannot challenge its validity. Our psychical experiences are, primarily, indeterminate magnitudes; they are incapable of exact treatment until they have been referred to determinate units of measurement, which in turn may be brought into constant causal relations with other given magnitudes. But we have, in the experimental modification of consciousness by external stimuli, a means to this very end – to the discovery of the units of measurement and the relations required. Modification from without enables us to subject our mental processes to arbitrarily determined conditions, over which we have complete control and which we may keep constant or vary as we will. Hence the objection urged against experimental psychology, that it seeks to do away with introspection, which is the sine qua non of any psychology, is based upon a misunderstanding. The only form of introspection which experimental psychology seeks to banish from the science is that professing self-observation which thinks it can arrive directly, without further assistance, at an exact characterization of mental facts, and which is therefore inevitably exposed to the grossest self-deception. The aim of the experimental procedure is to substitute for this subjective method, whose sole resource is an inaccurate inner perception, a true and reliable introspection, and to this end it brings consciousness under accurately adjustable objective conditions. (1874, p. 7)

Here, Wundt not only gives a (partial) response to Kant, but also lays out the plan for the whole of his psychology. First, Wundt makes a distinction between inner and outer experience, acknowledging that inner experience is indeed inaccessible to experimental methods. Indeed, what is generally known as Wundtian psychology, the kind that was undertaken in his laboratory at Leipzig and in which he trained nearly every member of the first generation of scientific psychologists, is his science of outer experience. Wundt’s psychology of inner experience, called Völkerpsychologie, had virtually no influence on later psychology (Leahey 2000). Völkerpsychologie (Wundt 1912) applies historical and anthropological methods of the kind Kant recommended in his Anthropology from a Pragmatic Point of View (1798). So Wundt accepts Kant’s arguments about inner experience, but argues that it is possible to experimentally access outer experience using the method of psychophysics. Psychophysics does not do away with introspection, but uses trained introspectors in highly controlled circumstances to experiment upon the basic elements of conscious experience. Subjects, for example, are presented with a series of tones that are increasingly high in pitch and are asked to report when they experience a pitch difference; or they are asked to press a key when a light appears on a screen; or they are asked to press when a blue light (but not a red light) appears on a screen. Experiments such as these are used to examine the variety of sensations humans are capable of having, and the temporal duration of sensory processes.

Using psychophysics to study outer experience places strict limits on the scope of experimental psychology. According to Wundt, psychology is the study of the elements of immediate conscious experience:

Psychological analysis leaves us with two such elements [of conscious experience], of specifically different character: with sensations, which as the ultimate and irreducible elements of ideas we may term the objective elements of the mental life, and with feelings, which accompany these objective elements as their subjective complements, and are referred not to external things but to the state of consciousness itself. In this sense, therefore, we call blue, yellow, warm, cold, etc., sensations; pleasantness, unpleasantness, excitement, depression, etc., feelings. (1874, p. 12)

Sensations and feelings are the atoms of conscious experience. They are combined by unconscious processes to form the molecules: sensations combine to form ideas; feelings combine to form complex feelings. Wundt’s experimental psychology was reductionist in that it focused on the smallest possible bits of conscious experience. It was also atomistic in that it thought that the elements maintained their identities in whatever context they appeared. (“As conscious contents, blue is and remains blue, and the idea of an object is always a thing ideated in the outside world, whether the external stimulus or the thing outside of us be really present or not” [1874, pp. 13–14].)

Wundt’s pioneering scientific psychology was, thus, a reductionist, atomistic science of outer experience. It was a successful science, and Wundt was very influential. He published more than 60,000 pages during his lifetime, and educated thousands of students in his methods (Fancher 1995). One of these students was William James, the “founding father” of American psychology. James later became an opponent of Wundt’s methods, which he mocked as “brass instrument psychology.” Another was E. B. Titchener, a proponent of Wundt’s methods who founded the psychology department at Cornell. In his 1898 textbook Titchener claimed to have identified more than 30,000 elementary visual sensations (1898b, p. 40). Most importantly for current purposes, Wundt’s was the scientific psychology that was in place when Husserl began his work as a phenomenologist. Husserl criticized Wundt specifically in establishing his phenomenological methods. Moreover, the psychological theories of the first half of the twentieth century that we will discuss later in the book defined themselves explicitly in opposition to Wundt’s views. The Gestalt psychologists, who were profoundly influential on Merleau-Ponty and on Gibson, rejected Wundt’s atomism (see Chapter 5). The functionalists and pragmatists, who were profoundly influential on Gibson, also rejected Wundt’s narrow focus on conscious experience (see Chapter 10).