The Influence of Physics and Chemistry on Psychological Theories

Psychology is often thought of as entirely separate from the so-called “hard” sciences. Yet some of the most important developments in psychological theory have come not from within psychology itself, but from physics, chemistry, and the engineering sciences. From the earliest attempts to measure sensation mathematically, to the discovery of neurotransmitters, to the rise of computational models of the mind – the physical sciences have fundamentally shaped how psychologists think about human behavior and cognition.

The birth of psychophysics: when physics met the mind 🔗

The story begins in the 19th century with a simple but radical question: can subjective experience be measured? Psychophysics, the study of the quantitative relationship between physical stimuli and the sensations they produce, emerged directly from this question. German physiologist Ernst Heinrich Weber was the first to take it seriously in the laboratory. In the 1830s, he discovered that the smallest detectable difference between two stimuli was not a fixed amount, but a constant proportion of the original stimulus – a finding that came to be known as Weber’s Law.

It was Gustav Theodor Fechner, however, who recognized the deeper significance of Weber’s findings. Trained in physics, Fechner in his later life became interested in metaphysics and searched for a way of relating the spiritual to the physical world, hitting upon the notion of measuring sensation in relation to its stimulus. His landmark 1860 book, Elemente der Psychophysik, formalized this relationship mathematically, stating that the perceived intensity of a sensation grows proportionally to the logarithm of the physical stimulus intensity. Psychophysics had an important immediate impact on psychology, sensory physiology, and related fields, because it provided a means of measuring sensation which previously, like all other aspects of the mind, had been considered private and immeasurable.

Psychophysics demonstrated that sensations and mental experiences can be quantitatively measured – a landmark discovery, as a number of earlier scholars had raised doubts regarding this. The methods Fechner introduced – including the method of limits and the concept of the absolute threshold – gave psychology its first truly scientific toolkit. Their work introduced methods for measuring the relationship between physical stimuli and human perception that would serve as the basis for the new science of psychology.

Weber’s Law and the Fechner scale 🔗

Weber’s Law can be stated simply: if you are holding a 10kg weight, you need to add roughly 1kg before you notice the difference; but if you are holding 100kg, you need to add around 10kg to notice the same change. The ratio stays constant. Fechner used this principle to derive a logarithmic scale – the Fechner scale – linking physical magnitude to perceived sensation. This mathematical formulation, borrowed directly from the tradition of physics, was the first time a psychological phenomenon had been expressed in a proper scientific equation. It became the conceptual cornerstone of experimental psychology.

Psychophysics and the founding of experimental psychology 🔗

Wilhelm Wundt, considered to be the founder of modern psychology, had envisioned the discipline of psychology to be the scientific investigation of consciousness. The quantitative measurement of sensations helped Wundt in achieving his vision. Wundt opened the world’s first experimental psychology laboratory in Leipzig in 1879, and the methods of psychophysics were central to his work there. Historically, psychophysics promised to make possible a quantitative and scientific study of what was then described as “higher mental processes,” now a topic in cognitive science. It had a direct effect on Binet, for example, who developed the now universally used methods for measuring intelligence.

The chemistry of behavior: neurotransmitters and psychological theory 🔗

While physics gave psychology its first methods of measurement, chemistry gave it a biological foundation for understanding why people think, feel, and behave as they do. The discovery that the brain is fundamentally a chemical system – one whose states can be altered by modifying molecular concentrations – transformed psychology from a science of introspection into a science grounded in biology.

Neurotransmitters are endogenous chemicals that allow neurons to communicate with each other throughout the body, enabling the brain to provide a variety of functions through the process of chemical synaptic transmission. Alterations in the levels of specific neurotransmitters have been observed in various neurological disorders, including Parkinson’s disease, schizophrenia, depression, and Alzheimer’s disease. Each of these conditions, which had long been described in purely psychological or behavioral terms, could now be understood as having a chemical substrate.

Key neurotransmitters and their psychological effects 🔗

Billions of neurotransmitter molecules work continually to keep the brain functioning, and to manage breathing, heartbeat, learning, and concentration. They affect a large variety of psychological functions, including fear, joy, mood, and pleasure. Among the most studied are dopamine, serotonin, and norepinephrine. Dopamine is often referred to as the “pleasure chemical” because it is released when mammals receive a reward in response to their behavior. It is one of the most extensively studied neurochemicals, mainly because it plays such diverse roles in human behavior and cognition – including motivation, decision-making, movement, reward processing, attention, working memory, and learning.

Serotonin, by contrast, is closely tied to mood regulation and emotional stability. Common neurotransmitters like serotonin, dopamine, and GABA play critical roles in mental health and emotional balance. Many psychiatric medications and recreational drugs work by altering neurotransmitter activity. This is precisely why selective serotonin reuptake inhibitors (SSRIs) – which prevent the reabsorption of serotonin in the brain – are now among the most prescribed medications for depression and anxiety disorders worldwide.

From chemistry to clinical psychology 🔗

The chemical understanding of behavior reshaped not just theory, but clinical practice. Once it became clear that psychological disorders could be linked to specific molecular imbalances, the pharmaceutical industry and clinical psychology began to converge. Serotonin is a hormone and neurotransmitter that helps regulate anxiety, appetite, mood, sleep, and sexual behavior. SSRI medications – which prevent the reuptake of serotonin in the brain to help balance levels – are often prescribed to treat anxiety disorders, depression, and panic attacks. Similarly, antipsychotic medications were designed around the dopamine hypothesis of schizophrenia, which proposed that excess dopaminergic activity in certain brain regions underlies psychotic symptoms. Chemistry, in short, gave psychology actionable leverage over mental illness.

Cybernetics, information theory, and the computational mind 🔗

Perhaps the most intellectually ambitious cross-disciplinary fertilization came in the mid-20th century, when ideas from engineering and mathematics began to reshape how psychologists thought about thinking itself. Two developments were especially important: cybernetics, the science of control and communication in animals and machines pioneered by Norbert Wiener, and information theory, developed by Claude Shannon in 1948. Together, they provided a new vocabulary for describing mental processes – one borrowed not from philosophy or biology, but from electrical engineering and mathematics.

Shannon’s information theory and Wiener’s cybernetics laid the groundwork for conceptualizing cognition as an interplay of information flows governed by feedback and control mechanisms. This framework was instrumental in the transition from behaviorist psychology to cognitive models that emphasized internal representations and processes. For the first time, the mind could be discussed not just in terms of behavior, but in terms of how information is encoded, stored, transmitted, and retrieved.

Cybernetics and the cognitive revolution 🔗

In the social and behavioral sciences, cybernetics included and influenced work in anthropology, sociology, economics, family therapy, cognitive science, and psychology. Norbert Wiener’s concept of feedback – the mechanism by which a system adjusts its behavior based on its own output – was directly applied to models of human goal-directed behavior. Psychologists began asking: could the mind be understood as a self-regulating feedback system, much like a thermostat or a guided missile? This question proved enormously generative.

Following the 1956 Dartmouth Symposium, cognitive psychologists became fascinated with machines that provided insight into storage, accession, and transformation of different forms of symbolic knowledge, strengthening information processing theory. Ulric Neisser suggested subjects understood information by creating mental representations that matched stimuli. George Sperling expanded Neisser’s ideas by investigating the storage capacity of iconic memory. These developments marked the beginning of the cognitive revolution – a paradigm shift in which internal mental processes, not just observable behavior, became the proper subject of psychological science.

The computational theory of mind 🔗

Ulric Neisser coined the term cognitive psychology in his book published in 1967, characterizing people as dynamic information-processing systems whose mental operations might be described in computational terms. This idea – that the human mind functions like a computer, processing inputs, storing data, and producing outputs – became the dominant framework in cognitive psychology for decades. Warren McCulloch and Walter Pitts were the first to suggest that neural activity is computational, arguing that neural computations explain cognition.

The computational theory of mind, as it came to be called, proposed that mental states are essentially representations, and that cognition consists in the manipulation of those representations according to rules. The decades following World War II saw the rise of cognitive science as a unified interdisciplinary enterprise, incorporating psychology, linguistics, computer science, philosophy, and neuroscience, driven by a shared interest in understanding the mind as a computational system. This cross-disciplinary convergence gave rise to fields like cognitive neuroscience, artificial intelligence, and computational psychiatry – all of which continue to flourish today.

Information theory’s specific contributions 🔗

Shannon’s information theory introduced a precise, mathematical definition of information, measured in bits. This allowed researchers to ask quantitative questions about human cognition: how much information can working memory hold? How fast does the brain process sensory input? The historical reason for the conflation of computation and information processing goes back to the cybernetic movement’s effort to blend Shannon’s information theory with Turing’s computability theory, as well as control theory. The result was a powerful theoretical framework that made it possible to model human cognition with mathematical precision, in the same way that physics models the behavior of particles or waves.

One concrete outcome was George Miller’s famous 1956 paper, which demonstrated that human short-term memory can hold roughly seven items (plus or minus two) – a finding phrased explicitly in the language of information theory. This kind of quantitative cognitive psychology would have been impossible without the conceptual tools imported from the physical and engineering sciences.

The enduring legacy of cross-disciplinary thinking 🔗

From Fechner’s logarithmic equations to the discovery of dopamine to the computational modeling of memory, the physical sciences have continuously expanded the explanatory reach of psychology. Each wave of influence – first from physics, then from chemistry, then from engineering and mathematics – pushed psychology toward greater rigor, precision, and empirical grounding. Psychophysics is not just a technique used in highly sophisticated studies of the senses, but rather a technique commonly used in many psychological measurements, not only in perception but also in realms as diverse as memory, learning, and social behavior. The same spirit of quantification that Fechner brought to sensation now operates across virtually every domain of psychological research.

Today, the conversation continues. Computational neuroscience uses mathematical models to simulate how networks of neurons give rise to perception, memory, and decision-making. Psychopharmacology refines our understanding of how specific molecules alter psychological states. And advances in neuroimaging allow researchers to watch the brain process information in real time, bridging the physical and psychological in ways Fechner could scarcely have imagined. The boundaries between psychology and the physical sciences have never been more productively blurred.

What do you think? Given how deeply physics, chemistry, and computing have shaped our understanding of the mind, does this make psychology more or less of an independent science in your view? And as artificial intelligence grows more sophisticated, do you think computational models will eventually offer a complete account of human cognition – or will something always remain beyond the reach of equations?

References

1. https://www.britannica.com/science/psychophysics
2. https://en.wikipedia.org/wiki/Psychophysics
3. https://plato.stanford.edu/entries/computational-mind/