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How did 19th-century materialism challenge belief in a soul or mind separate from the body?

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Fragment of a Roman fresco showing Socrates seated and draped in a white robe, with Greek text above, on a red background with visible cracks. — Philosophical anthropology Socrates, Roman fresco, 1st century bce; in the Ephesus Museum, Selçuk, Turkey.

During the 19th century, the traditional concept of the soul or mind as an entity distinct from the body faced a radical challenge from materialism, which sought to explain human nature and mental functions through the authority of the natural sciences.1

This shift was driven by several key developments:

Scientific Reductionism: As vital functions once attributed to the soul were increasingly explained by physical and physiological processes, the necessity of a separate "soul" became problematic. Scientific thinking began to recognize facts only when they could be reduced to a common level of physical process.21
Biological and Chemical Advances: The 1828 synthesis of urea bridged the gap between organic and inorganic chemistry, undermining the idea of a unique "vital" force. Later, Charles Darwin’s theory of evolution provided a purely causal basis for human existence, showing continuity between humans and other living things.3
Mechanistic Theories: 19th-century thinkers like Karl Vogt and Ludwig Büchner proposed that thought was a product of the brain—sometimes described as a physical motion or a secretion—effectively challenging the dualistic view that separated mind from matter.3

These developments forced a crisis in how humans understand themselves, leading to a consensus in many scientific and philosophical circles that human nature must be understood within a materialist ontology.1