Philosophically considered, the universe is composed of Nature and the Soul. Strictly speaking, therefore, all that is separate from us, all which Philosophy distinguishes as the 'Not Me,' that is, both nature and art, all other men and my own body, must be ranked under this name, 'Nature.'

This quote is suggesting that the universe is made up of two parts: Nature and the Soul. Nature can mean anything separate from us, so it includes human-made things, other people, and our own bodies. All of these things can be viewed as part of Nature, and the Soul is separate from them. Therefore, Nature includes anything that is not part of the Soul.