Prior to the 19th century, psychology was considered a branch of speculative philosophy. It was not recognized as an independent science with its specific subject matter, and methods of study. The earliest Greek philosophers were of the opinion that psychology was not different from philosophy. The development of psychology as a distinct branch of knowledge was very much influenced by Greek philosophical thought. The early Greek philosophers described psychology as the science of soul. They tried to explain how the soul functions, where it exists, whether soul can be seen, and what happens to the soul after death. The ancient people believed that soul exists even after the death of the man; soul is invisible, but immortal.

The Greek philosophers like Plato and Aristotle (384 BC – 322 BC) tried to explain the nature of soul, but they never tried to make it a subject matter of science, and hence, never tried to establish psychology as a science. Plato explained the concept of psychology as the “science of soul”, which is unscientific.

Plato believed that soul has no physical existence. As the soul can neither be seen nor touched, nor its existence scientifically demonstrated, his concept of psychology was not accepted. However, his ideas distinguished ‘soul’ from the ‘body’. Aristotle was of the opinion that mind is the function of the body. He believed that heart is very much related to the activities of human beings. The objects in the environment stimulate the sense organs and the results of such stimulation are transmitted to the heart. His view was to some extent correct as he said that the mental functions are the products of physical entity.

The contribution of Rene Descartes (1596-1650) was important for the development of psychology. He distinguished between mind and body, but recognized that each influences the other. He believed that the animals do not have soul, and therefore, behave like machines. The animals are “automata” as their activities are mechanical. But man has a soul, which controls his activities. The soul, as he believed, exists in the pineal gland, which is located at the base of the brain. So his concept of soul was different from that of Aristotle. His “dualism” explains that the mind and body are different; yet there is interaction between both. The interaction takes place in the pineal gland. But his concept could not explain how the interaction takes place.


The physiologists questioned his idea that the interaction between the mind and the body takes place in the pineal gland. Another contribution of Descartes having a direct influence on psychology was his belief in innate ideas. He believed that there are certain truths or “axioms” which provide the basis for demonstrable  knowledge, and that these truths are inherent.

After Descartes, psychology was still treated as a branch of philosophy for a long period. During the period lasting for about two centuries, the physiologists and physicists contributed immensely to the understanding of human behavior. These contributions laid the foundation of psychology as a science. Among the later contributions, the most important were those of Thomas Hobbes, John Locke, David Hume, David Hartley, Helmholtz, Heinrich Weber, and G.T. Fechner.

Weber and Fechner studied the relationship between physical stimulation and psychological experience using psychophysical methods, and propounded quantitative laws. Helmholtz measured the speed of nerve currents, and proposed theories of color vision and audition. His attempts to the study of psychology were more systematic and organized. In spite of all the progress made before the mid-nineteenth century, psychology continued to exist as a branch of mental philosophy. It was not treated as a separate discipline as its subject matter, objectives, methods, and applications were not clearly defined. There was no formal agreement among the early founders with respect to what this new science should be.

The philosophical thoughts during the eighteenth and nineteenth centuries provided the basic ground rules for the development of modern psychology. At the same time, the proponents of natural science argued that the systematic methods of formal sciences could be applied to study many aspects of human behaviour.


Charles Darwin, a British scientist, published a book “The Origin of Species” in 1859, which revolutionized thinking in the fields of biology and genetics. He placed human beings and animals on a continuum. Human beings like other organisms, change as they adapt to their natural environment. As such, they share some common characteristics, and much can be learned about human behaviour by observing the behaviors of other organisms.

The methods of science applied to the study of other organisms can also be applied with human beings.The modern psychologists have shown interest in studying the behaviors of rats, cats, dogs, chimpanzees, and other animals, and have made inferences about human behavior.