Final answer:
Descartes, as an idealist, believed in the independence of the mind from physical substance, while as a materialist, he acknowledged the existence of physical entities like trees, colours, and electrons as mind-independent.
Step-by-step explanation:
René Descartes was both an idealist and a materialist, but not a dualist. For Descartes, the mind is independent and does not need physical substance; it is non-material and capable of thought, imagination, hope, dreaming, and more without the body. Conversely, trees, colours, electrons, and other physical entities are mind-independent in materialism. They exist as materials or physical substances within the realm of matter.
However, Descartes' 'Cogito, ergo sum' suggests that our own existence is known through thought alone, distinguishing the mind as a different sort of substance than the body. While the materialist view denies the existence of non-physical minds, stating that all mental activities are brain functions, the idealist perspective asserts that all that exists are ideas, and that physical perception may be just an illusion coordinated by a divine creator.