The word psychology comes to us from the Greek word psyche, meaning soul or spirit, and logos, meaning explanation. In fact, the history of psychology as a science is rooted in ancient Greece.
Philosophers such as Plato and Aristotle conducted philosophical inquiries into the nature of the human soul. In his book The Republic, Plato put forth his theory that experience could not affect essential aspects of human nature. Aristotle, on the other hand, placed the origins of human behavior on lived experiences.
Though these philosophers never went as far as producing a scientific discipline out of these theories, their work provided a philosophical blueprint to establish psychology as a science down the line. Philosophers like René Descartes, Jean-Jacques Rousseau and John Locke also laid much of the philosophical groundwork that would be formalized into psychology as science.
Beginning in the mid-1800s, theories of the mind and human behavior began to formalize into a real scientific discipline. The German physicist and physician Hermann von Helmholtz, for instance, explored how hearing and vision are interconnected and how senses can be deceived. By creating experiments to prove the fallibility of human senses, Helmholtz helped develop the discipline of psychophysics, a precursor of psychology as a science.
Throughout the century, psychologists worked to formalize the discipline and introduce scientific methodology. Formal education in the subject was developed, along with textbooks and experimental research. Experiments were performed on things like senses, reaction times and perceptions. These produced data that could be experimentally reproduced, published and analyzed.
Though many psychologists disagreed on the exact origins of human behavior, and though many subdisciplines and theories moved in and out of prominence, it was during this time that psychology was formalized as a science.
Moving into the 20th and 21st centuries, many psychological theories were attested, evaluated, proved, disproved and revised. In all, the scientific method has been used to produce theories that are useful and widely applicable.