Materialist philosophy is predicated on the idea that only matter truly exists and that all phenomena result from the interactions of matter. To the materialist, concepts such as the soul and consciousness thus have no role in defining reality. As such, materialism is a monist philosophy (believing that everything is comprised of matter alone) and stands in contrast to idealism (see next entry here).
Although he post-dated the earliest materialist traditions (which existed in India from around the 6th century BCE, and in Greece where it was variously propounded by, among others, Thales, Democritus and Epicurus), the great Roman thinker Lucretius wrote the first widely known treatise on materialism around 50 BCE. In De rerum natura (On the Nature of Things), he explicated the idea of atomism (see here).
For a great many centuries, the idea of materialism fell out of favour across much of the world, with the rise of the Abrahamic religions fuelling belief that there exists an immaterial world separate from the material one. However, the onset of the Enlightenment revived interest in the concept. René Descartes famously investigated the mind–body problem (see entry on Rationalism here), with his resulting philosophy of Cartesian dualism arguing that the mind and the body are two different entities. This was in opposition to the ideas of Thomas Hobbes, who had suggested all human experience derives from mechanical processes within the material body.
While Cartesian dualism allowed for the existence of a soul (and thus could be accommodated within the traditions of Christian teaching), another Frenchman, Baron d’Holbach (1723–89), took a radical atheistic, materialist line, claiming that everything that happens is the result of matter and its motion. In his masterpiece, The System of Nature (1770), he wrote:
Man’s life is a line that nature commands him to describe upon the surface of the earth, without his ever being able to swerve from it, even for an instant . . . he is unceasingly modified by causes, whether visible or concealed, over which he has no control, which necessarily regulate his mode of existence, give the hue to his way of thinking, and determine his manner of acting. He is good or bad, happy or miserable, wise or foolish, reasonable or irrational, without his will being for any thing in these various states.
The German physician Ludwig Büchner (1824–99), meanwhile, wrote Kraft und Stoff: Empirisch-naturphilosophische Studien (Force and Matter: Empiricophilosophical Studies) in which he argued the materialist case from a purely scientific perspective. Indeed, the broad scientific developments of the 19th and 20th centuries – such as evolution, neuroscience and atomic theory – generally bolstered the materialist position, while the role of the spirit and theology gradually lost leverage among much of the scientific community.