XXXVI, 1, 2026: The Psychophysical Adventure. From Wolff to Behaviorism. Edited by Chiara Russo Krauss and Giuseppe Guastamacchia

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The rise of modern science was marked by a shift from the knowledge of qualities to the knowledge of quantities: the old ontology of essences gave way to an understanding of nature based on mathematized laws. Physics came to embody this approach in its fullest form, thereby becoming a powerful epistemic model for other disciplines. However, since this approach rested on the investigation of fundamentally quantitative elements – bodies and their motion – the question remained whether it could be extended beyond the domain of physical nature. The psyche, in particular, seemed to belong to another order altogether: as the realm of consciousness, it was regarded as almost by definition non-spatial, immaterial, and uncountable.
The term “psychophysics” therefore contains something almost paradoxical, bringing together in a single word two domains that were often considered irreconcilable. As such, it expressed a challenge: the bold claim that the gap separating knowledge of the physical from knowledge of the mental could be bridged, and that the rigorous paradigm of mathematized lawfulness could be extended to the domain of consciousness.
Therefore, even though the term “psychophysics” entered the public stage in 1860 with Gustav Theodor Fechner’s Elemente der Psychophysik, it names a much longer tradition: one in which the development of a mathematical understanding of nature repeatedly encouraged attempts to bring the mental domain within the scope of physical knowledge.
As early as the eighteenth century, Christian Wolff’s Psychologia empirica (1732) and Philosophia practica universalis (1738-39) laid the foundations for a psychometria conceived as mathematical knowledge of the human mind. Later in the century, Johann Gottfried Herder, Johann August Eber-hard, Johann Nicolaus Tetens, and Friedrich Schiller debated the continuity between physical stimulus and psychic sensation, drawing on the work of scientists such as Albrecht von Haller and Johann Heinrich Lambert.
These debates prompted Kant, in the Metaphysical Foundations of Natural Science (1786), to ask whether psychology could ever become a mathematized form of knowledge on a par with physics. His answer was negative: psychology, he argued, could not be a proper science because inner experience lacks spatial extension. Yet, despite being presented with Kant’s usual air of finality, this verdict did not put an end to the issue. Rather than serving as a tombstone for the project of a physics of the psyche, it acted as a renewed challenge for those thinkers who sought to show, against Kant, that the mind could become the object of mathematical knowledge. In the long run, the brief discussion of the “Anticipations of Perception” in the Critique of Pure Reason, where Kant argues that sensations possess degrees of intensive magnitude, proved more influential than his explicit denial, in the Metaphysical Foundations of Natural Science, of the possibility of mathematizing psychology.
During the nineteenth century, Kant’s “interdict” against the mathematization of inner life was contested from several directions. Among the earliest and most influential responses was Johann Friedrich Herbart’s psychical mechanics, which sought to treat mental representations as quantitative forces acting dynamically upon one another, thereby making possible the formulation of mathematical laws analogous to those of physics. His Psychologie als Wissenschaft (1825) introduced notions such as representational force, threshold of consciousness, degrees of inhibition (Hemmung), and repression (Verdrängung), which would provide the conceptual infrastructure not only for direct followers of Herbartian psychology, such as Moritz Whilelm Drobisch and Ludwig Strümpell, but also for Fechner’s psychophysics and, eventually, Sigmund Freud’s psychodynamics.
Whereas Herbart’s program sought to mathematize the mind by treating representations and their internal dynamics directly in quantitative terms, without appealing to their neural or physical substrate, Fechner’s psychophysics was conceived from the outset as the exact doctrine of the relations between soul and body. For Fechner, the intensity of sensation could be expressed mathematically as a function of the corresponding intensity of physical stimuli, and, more generally, the intensity of mental activity could in principle be expressed as a function of the corresponding physiological or neural activity. Thus, in the thinker with whom psychophysics is most closely associated, the term already has a somewhat double meaning. On the one hand, it designated a quantitative knowledge of the mind that leads to the formulation of mathematical laws on the model of physics. On the other hand, it designated the study of the mind from the standpoint of its connection with the neural substrate and under the assumption of a close parallelism between psychical activity and its bodily correlate.
In both senses, psychophysics was also closely connected to the contemporaneous development of experimental and physiological psychology. Unlike Herbart’s mental mechanics, which remained largely a theoretical endeavor, the first breakthroughs in psychophysics, such as the formulation of the Weber-Fechner law, resulted from experimental investigations. At the same time, the study of the sensory apparatus carried out by Johannes Peter Müller and his school, including Hermann von Helmholtz and Emil Du Bois-Reymond, paved the way for a scientific study of the mind based on rigorous, quantitative, experimentally acquired data, and closely linked to nervous processes.
Even thinkers such as Helmholtz and Wilhelm Wundt, who emphasized that mental content has a qualitative and creative dimension irreducible to measurement and inaccessible to an approach modeled entirely on physics, continued to rely on experimental methods, quantitative data, and physiological perspectives. Wundt’s reaction-time experiments, for instance, placed introspection within a controlled experimental setting and a quantitative framework, but they did so by relocating the central problem of psychophysics: rather than measuring mental contents directly, they measured the temporal conditions under which conscious processes unfold.
In fact, the crucial debate surrounding psychophysics revolved around the measurability of mental content. On the one hand, several thinkers developed different strategies for measuring psychical processes: Herbart treated representations as forces; Fechner relied on just noticeable differences; Wundt turned to reaction times; and similar attempts sought to bring mental life within a quantitative framework. By contrast, other thinkers insisted on the irreducibly qualitative character of conscious activity. Wilhelm Dilthey, for instance, emphasized the difference between the quantitative structure of physical processes and the hermeneutic character of lived experience, thereby arguing for a sharp distinction between the natural sciences and the Geisteswissenschaften. Franz Brentano, in Psychologie vom empirischen Standpunkt (1874), argued that psychophysics does not really measure mental phenomena in any strict sense, but only physical contents – colours, sounds, degrees of warmth – and thus systematically neglects the proper nature of psychical phenomena, namely their intentional relation to an immanent object. Carl Stumpf, Edmund Husserl’s early mentor and himself deeply indebted to Brentano, pursued a phenomenological analysis of tonal qualities that challenged the reduction of sensory experience to merely quantitative differences. Beyond Germany, Henri Bergson’s Essai sur les données immédiates de la conscience (1889) offered a radical critique of the psychophysical program. What we call a greater or lesser intensity of sensation, Bergson argued, is not a quantitative difference between homogeneous units, but a qualitative transformation of lived experience. Psychophysics becomes possible only by spatializing consciousness, forcing the fluid continuity of inner life into external, numerical, and measurable terms.
As had already happened with Kant’s interdict, these polemics did not lead to a retreat from the challenge of psychophysics. Rather, they led to a deepening, refinement, and reformulation of the project of a quantitative scientific study of the mind, opening new ways of thinking the relation between the mental and the physical. New trends in experimental psychology sought to combine rigorous methods with a less reductive understanding of mental life, for instance through questionnaires, introspective protocols, and statistical analysis. Oswald Külpe’s Denkpsychologie used such tools to investigate higher mental functions experimentally. Gestalt psychology as well, while remaining firmly rooted in experimental research, challenged the elementalist assumption that perception could be explained through a constant relation between local stimulation and elementary sensation, conceiving it instead as an organized whole governed by its own structural laws. Behaviorism represented an even more radical solution: if consciousness could not be made into a publicly observable and experimentally controllable object, scientific psychology could redefine its domain altogether, focusing not on inner experience but on observable behavior and the relations between stimuli and responses.
However, the debate around psychophysics did not only foster important developments in the history of psychology; it also had a broader philosophical significance. It was never merely a set of problems belonging to the embryonic phase of new scientific disciplines, and therefore destined to be solved by later advances in experimental psychology, physiology, or neuroscience. Rather, it brought to the surface, and reformulated in new terms, a series of enduring philosophical problems concerning the distinction and relation between physical and psychical phenomena.
Therefore, many of the questions debated by the thinkers involved in the development of psychophysics are still with us today. For this reason, revisiting these debates is not merely of historical interest: it helps us understand why such questions persisted over time, and how they were gradually reformulated as advances in psychometrics and neurophysiology transformed the scientific study of the mind.
These problems concern, first of all, the very meaning of measurement when applied to mental life. What exactly is measured in psychophysics? Is it sensation itself, or only the physical stimulus that gives rise to it? Is it a psychical magnitude, a physiological correlate, a reaction time, a threshold of discrimination, or a behavioral response?
A second problem concerns the relation between the physical and the psychical, which could be understood as continuity, correspondence, functional dependence, parallelism, causal interaction, or irreducible qualitative difference. If the mental is continuous with or analogous to the physical, then it seems open to mathematization; if it merely corresponds to it, the question becomes how to coordinate two different orders of phenomena; if it is qualitatively irreducible, then psychophysics risks losing sight of the true character of mental processes.
Closely connected to this is the problem of reduction. Can mental phenomena be explained in scientific terms without being dissolved into physiology? Can they be studied as embedded in the natural order while still preserving what makes them mental: their qualitative character, their intentionality, their subjective givenness, and their relation to freedom and meaning? This question made psychophysics a crucial testing ground for broader disputes over materialism, parallelism, dualism, and physicalism.
A further problem is that of elementarism. The possibility of measuring mental content was closely connected to the assumption that mental life could be analyzed into fundamental units. Despite recurring polemics against the increasingly antiquated model of associationist psychology, nineteenth-century attempts to place the study of the mind on scientific grounds continued to go hand in hand with the search for its basic elements. This tendency, in turn, provoked renewed criticism from thinkers who emphasized that the mind is a domain in which the whole determines the meaning and function of its parts, rather than the other way around.
Running through all these issues is the problem of method. If mental life could not be captured by measurement, or by measurement alone, what other procedures were required? Experiment, controlled introspection, questionnaires, phenomenological description, and hermeneutic understanding all emerged as competing or complementary ways of approaching the mind. The history of psychophysics is therefore not only the history of an attempt to quantify consciousness, but also the history of the methodological pluralism that arose from the resistance of consciousness to quantification.
The persistence of these problems was accompanied by the persistence and circulation of the concepts forged to address them. Psychophysics did not merely raise questions that remained philosophically alive; it also produced a vocabulary that migrated across disciplines and acquired new meanings in different contexts. Herbart’s notion of inhibition (Hemmung), originally introduced to describe the dynamics of conflicting representations, was later transformed in neurophysiology and Freudian metapsychology. The concept of threshold (Schwelle) followed an equally significant trajectory: from Herbart’s threshold of consciousness to Fechner’s limen of psychophysical measurement, it became a central device for articulating the relation between stimulus, sensation, consciousness, and unconscious processes, and it continues to re-emerge, in transformed forms, in neurophysiological, cognitive, and computational models that rely on mechanisms of activation and threshold. The history of psychophysics is therefore also the history of concepts whose theoretical afterlives exceeded the intentions of their first authors.
The essays collected in this volume approach this history and these problems from different but converging perspectives, combining conceptual analysis with the history of philosophy and the history and philosophy of science. Taken together, they show that psychophysics was not simply an episode in the beginnings of modern psychology, but a crucial point of tension within modernity itself. Modern science had increasingly defined itself through quantification and the search for mathematized laws; modern philosophy, by contrast, had placed the human mind, consciousness, and the knowing subject at the center of its concerns. Psychophysics emerged precisely at the intersection of these two trajectories. It asked whether the quantitative paradigm that had transformed the knowledge of nature could be extended to the very domain that modern philosophy had made its own: inner experience, consciousness, and subjectivity.
For these reasons, psychophysics is not a closed chapter. It is, rather, a field in which philosophy, psychology, physiology, and the history of science continue to meet, and whose questions, far from having exhausted their theoretical force, still demand sustained and systematic inquiry.

Contents
(click on the titles to view the abstracts)

Chiara Russo Krauss, Giuseppe Guastamacchia, Introduzione
Paul Ziche, ‘Thresholds’: From Psychology and Psychophysics to the Human
Micaela Bellizzi, La psychometria tra ontologia, psicologia e pratica: Wolff e la scuola wolffiana alla soglia tra gradus e mensura
Ruben Norloos, The First Parallelist Interpretations of Spinoza
Riccardo Martinelli, Fechner, Spinoza e il superamento del parallelismo
Guillaume Fréchette, Psychophysics and Its Discontents: Fechner and the School of Brentano
Chiara Russo Krauss, Fechner and Wundt on Conservation of Energy and Free Will
Juan David Millán, “Physiologists Had Been Mistaken or Had Even Strayed From the Path”. An Analysis of Early Wundt’s Critiques of Induction in Physiology and Its Philosophical Principles (1858-1866)
Giuseppe Guastamacchia, From Psychomechanics to Freudian Theory: Ludwig Strümpell on Physical Stimuli and Dream State
Basil Vassilicos, Gradients of Perceptual Noise: Considerations from Karl Bühler’s
Lino Rossi, Annibale Pastore fra logica e psicofisica: il calcolo psicofisico a tre variabili
Brenda Cardoso Soares, Constancy Hypothesis: Psychophysics and Gestalt
Luca Guidetti, Aspetti fenomenologici nella psicologia della forma: il problema interno-esterno