The "hard problem of consciousness" refers to the challenge of explaining why and how subjective experiences arise from physical processes in the brain. While scientists can study brain activity and identify correlations with behaviors, they struggle to understand how these neural mechanisms translate into personal experiences, such as feelings and perceptions. This concept was popularized by philosopher David Chalmers in the 1990s. He distinguished between the "easy problems" of consciousness, which involve understanding functions and behaviors, and the "hard problem," which focuses on the nature of experience itself. This distinction highlights the complexity of understanding consciousness fully.