All existence is eternal. All substance is infinite. This is truly only one substance, since no two substances can share the same attributes. Spinoza, a Dutch philosopher, held the view that mind and body were just two aspects of the same one reality. This is pantheistic, or the belief that all is God.
Mental reality is subjective or internal, and bodily reality is objective or external.
We know our inner life intimately, but we only know the physical world second-hand through sense experience. Spinoza was a rationalist. For Spinoza, the mind and the body are aspects of the Absolute. The mind is the thinking mode, and the body is the extended mode. Hence, there isn't a separation of mind-body for Spinoza like there was for Descartes' dualism.