The philosopher described is René Descartes.

Why Descartes fits the description

Descartes is famous for mind–body dualism , the view that:

  • The mind is a non-physical, thinking substance.
  • The body is a physical, extended substance.
  • Each can exist independently of the other, even though in a living human they are united as one whole.

In other words, for Descartes the mind and body are really distinct : the mind can be clearly conceived without the body and the body without the mind, so they are separate kinds of things that nevertheless interact and form a single human being.

A quick way to remember this: whenever you see a question about the mind and body being two different substances that can exist apart but currently function together, it is almost always pointing to Descartes and Cartesian dualism.

TL;DR: The answer to “which philosopher believed the mind and body are mentally independent in function even if they exist together as a whole?” is René Descartes.