From Soul to Science: Tracing Psychology's Early Definitions • Psychology Town

Psychology is one of the most dynamic sciences in human history – not just because of what it studies, but because it has repeatedly redefined what it is. The word itself comes from the Greek psyche (soul or mind) and logos (study). But beyond etymology, the actual definitions psychologists have used to describe their own discipline have shifted dramatically over the centuries – from metaphysical ideas about the soul, to the private world of the mind, to the measurable world of observable behavior. Understanding these early definitions isn’t just a history lesson; it reveals how scientific thinking matures and refines itself over time.

Psychology as the “science of the soul”
The shift to the “science of mind”
Herbart and the problem of scientific method
The “science of consciousness” and the birth of the laboratory
Why introspection fell short
The behaviorist turn: psychology as the “science of behavior”
What behaviorism gained – and what it left out
What this history tells us

Psychology as the “science of the soul”

The earliest conception of psychology was deeply rooted in philosophy and religion. The word “psychology” first appeared in its Latin form, psychiologia, in a work by Croatian humanist Marko Marulić around 1510-1520, and for a long time, the discipline it described was essentially an inquiry into the human soul. In ancient Greece, thinkers like Plato and Aristotle dominated this conversation. Plato argued that the soul was the essential core of a person, shaping both thought and action. Aristotle took a more functional view, treating the soul as the animating principle of a living being, with distinct capacities for intellect, emotion, and desire.

This framing – referring to psychology as the scientia de anima, or science of the soul – persisted for centuries. It wasn’t a fringe view; it was the established intellectual tradition. Scholars working within this framework weren’t asking whether the soul existed, but rather what properties it had and how they could be understood. Before the 18th century, psychology existed essentially as a “physics of the soul,” belonging as much to natural philosophy as to Christian anthropology.

The critical limitation of this definition, however, was obvious: the soul could not be directly observed, measured, or tested. It was a metaphysical concept, and no matter how rigorously philosophers reasoned about it, they couldn’t subject it to experimental scrutiny. This tension would eventually force a redefinition.

The shift to the “science of mind”

By the 17th and 18th centuries, the Aristotelian framework began to fracture. Philosophers started moving the conversation away from the soul and toward a more concrete concept: the mind. Key figures in this transformation were Descartes, Locke, and Wolff, each of whom contributed to reshaping what psychology was thought to study.

René Descartes introduced his famous mind-body dualism, arguing that the mind – as a “thinking substance” – was fundamentally distinct from the physical body. While this created its own philosophical problems (how exactly do mind and body interact?), it was a step toward treating mental activity as something distinct and worthy of separate study. John Locke pushed further still. His empiricist philosophy proposed that the mind begins as a blank slate (tabula rasa) at birth, with all knowledge derived from experience. This was significant because it implied the mind could be studied through its products – perceptions, memories, and ideas – rather than through abstract metaphysical claims.

In Germany, Christian Wolff formally identified psychology as its own science, writing Psychologia Empirica in 1732 – an early landmark in establishing the discipline as an independent field of inquiry, not merely a branch of theology or philosophy. This Enlightenment-era refashioning transformed the “science of the soul” into the “science of the mind.”

The concept of mind was more tractable than the soul – it had identifiable functions like memory, perception, and reasoning. But it was still an internal, private entity. You couldn’t directly observe someone else’s mind. This left psychology in a difficult position: it had a more scientific-sounding subject matter, but no reliable method for studying it from the outside.

Herbart and the problem of scientific method

The 19th-century philosopher and psychologist Johann Friedrich Herbart grappled directly with this problem. According to Herbart, psychology needed to be based on empirical research and mathematics, yet he also believed it could not be a fully observational science – because mental states, unlike physical objects, couldn’t be directly inspected from the outside. His position highlighted a core tension: psychology wanted to be a science, but its subject matter resisted the methods that made other sciences work.

The “science of consciousness” and the birth of the laboratory

Psychology’s most decisive step toward becoming an empirical discipline came in 1879, when Wilhelm Wundt founded the first experimental psychology laboratory at the University of Leipzig in Germany. This event is widely regarded as the formal beginning of psychology as an independent scientific discipline, separate from both philosophy and physiology.

Wundt’s focus was consciousness – the immediate, lived experience of perception, sensation, and feeling. When psychology was viewed as the scientific study of conscious experience, Wundt developed methods such as introspection to objectively measure its components. Introspection, in Wundt’s framework, wasn’t casual self-reflection – it was a highly structured, trained technique. Participants would be given a stimulus such as a ticking metronome and asked to report their thoughts and feelings in response, under carefully controlled and repeatable conditions.

Wundt’s approach was genuinely groundbreaking. He brought psychology into the laboratory, gave it instruments, measured reaction times to the nearest millisecond, and insisted on replicable procedures. His main goal was to promote psychology as a distinct scientific discipline apart from philosophy, demonstrating the importance of experimental methods and systematic observation in studying mental processes.

Why introspection fell short

Despite Wundt’s rigor, defining psychology as the science of consciousness carried a fundamental weakness: introspection produced subjective data. Despite efforts to train individuals in the process of introspection, the method remained highly subjective, and there was very little agreement between individuals. If two trained observers reported different experiences in response to the same stimulus, which one was correct? There was no external standard by which to judge.

William James, often called the father of American psychology, also worked within the science-of-consciousness framework, famously describing the continuous flow of mental life as the “stream of consciousness.” But even James acknowledged the difficulty of studying inner experience in a truly scientific way. Defining psychology around consciousness kept it tied to the subjective, the private, and the ultimately unverifiable. Critics argued that no matter how controlled the lab conditions were, introspective data couldn’t meet the standards of objective science. The stage was set for a radical rethink.

The behaviorist turn: psychology as the “science of behavior”

The most consequential redefinition in psychology’s early history came in 1913, when American psychologist John B. Watson delivered a lecture at Columbia University titled Psychology as the Behaviorist Views It – what later generations would call the “behaviorist manifesto.” In this lecture, Watson mounted a direct attack on the mainstream definition of psychology as the science of mind or consciousness, arguing in clear terms that psychology should be viewed as a purely objective experimental branch of natural science whose theoretical goal is the prediction and control of behavior.

For Watson, consciousness was not a legitimate scientific concept. You couldn’t see it, point to it, or measure it from the outside. Introspection, he argued, was unreliable and unscientific. His groundbreaking article rejected the then-dominant method of introspection, arguing instead that psychology should study only observable actions and measurable responses. What you could study, Watson insisted, was behavior – the actions that organisms perform in response to stimuli, actions that could be observed, recorded, and replicated by any researcher with the right equipment.

Watson was more an exemplar of a movement than a lone founder – behavioral methods had been developing for decades before his 1913 paper. But his manifesto gave the emerging approach a clear and combative identity, and it landed at precisely the moment when psychology was ready to embrace it. The definition of psychology as the “science of behavior” quickly became dominant in American psychology throughout the early-to-mid 20th century, reinforced by later behaviorists like B.F. Skinner.

What behaviorism gained – and what it left out

Behaviorism solved psychology’s credibility problem in one stroke. By restricting itself to observable behavior, the field could apply the same standards of objectivity, measurement, and replication used in physics or chemistry. Pavlov’s work on conditioned reflexes in dogs, Watson’s own experiments on learned emotional responses, and Skinner’s operant conditioning research all demonstrated that behavior could be studied with remarkable precision.

But behaviorism’s strict rejection of mental states also drew criticism. By excluding thoughts, feelings, intentions, and consciousness from scientific inquiry, it left large swaths of human experience unexplained. The cognitive revolution of the 1950s and 60s would eventually push back, restoring mental processes to the center of psychological research. Today, psychology is defined as the scientific study of the mind and behavior, encompassing both conscious and unconscious phenomena, mental processes, and the behavior of humans and nonhumans alike. The modern definition is, in many ways, a synthesis – combining the empirical rigor behaviorism demanded with renewed attention to the inner life that the science of consciousness had tried to explore.

What this history tells us

The evolution from “science of the soul” to “science of behavior” isn’t just a story of academic definitions changing. It reflects a broader struggle to make the study of human experience genuinely scientific – to find methods rigorous enough to produce reliable knowledge about something as complex and elusive as the mind. Each definition solved some problems while exposing others. The soul was too metaphysical to measure. The mind was more concrete, but still private. Consciousness could be studied in a lab, but the data remained stubbornly subjective. Behavior was finally observable – but focusing exclusively on it stripped psychology of much of what made human beings interesting in the first place.

This iterative process of definition, critique, and redefinition is exactly how science progresses. Psychology didn’t get its definition wrong and then finally get it right. It got progressively clearer about what it could and couldn’t study, and how.

What do you think? Given that each early definition of psychology addressed the limitations of the one before it, do you think the current definition – as the science of both mind and behavior – has fully resolved those tensions, or are there still aspects of human experience that fall outside psychology’s scientific reach? And if the field continues to evolve, what might the next major redefinition look like?

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Table of Contents

Psychology as the “science of the soul” 🔗

The shift to the “science of mind” 🔗

Herbart and the problem of scientific method 🔗

The “science of consciousness” and the birth of the laboratory 🔗

Why introspection fell short 🔗

The behaviorist turn: psychology as the “science of behavior” 🔗

What behaviorism gained – and what it left out 🔗

What this history tells us 🔗