Why did Martin Heidegger reject the idea that humans are isolated minds?

Martin Heidegger rejected the idea that humans are isolated minds—a view he associated with the Cartesian tradition—because he argued that the fundamental character of human existence is "being-in-the-world."123

Rather than viewing the individual as a "thinking subject" or "mind" that is radically distinct from and cognitively isolated from the material world, Heidegger posited that human beings are inherently engaged with their environment from the start. He introduced the term Dasein (literally "being-there") to replace traditional concepts like "consciousness" or "mind," emphasizing that humans are always already caught up in, involved with, and committed to other individuals and things.123

According to Heidegger, this practical, communal engagement with the world is ontologically more basic than the theoretical, individualistic perspective of a detached observer. By defining human existence as "being-in-the-world," he dismantled the assumption that specific mental states are required to bridge a gap between an isolated subject and an external world, as the world is already presupposed by human experience.341