What did Claude Bernard mean by the body’s internal environment?

The French physiologist Claude Bernard introduced the concept of the milieu intérieur, or "internal environment," to describe the fluid environment within an organism that surrounds its cells and tissues.

Rather than being subject to the fluctuations of the external world, this internal environment remains relatively stable, allowing the organism to maintain its vital processes despite external changes. Bernard’s work on this concept was foundational to the modern understanding of homeostasis, which refers to the self-regulation of these internal physiological processes. By establishing that life is mechanistically determined by physico-chemical forces, Bernard argued that the stability of this internal environment is essential for the independence and survival of the organism.12