Res externa and res cogitans are the two substances of the universe, according to Descartes. The world consists of (1) matter and (2) mind.

Consider your body. It is in a space of tangible things that exist. This is the world of matter.

Now attend to your “inner” experience—the one that others do not share. Your thoughts, bodily sensations, desires, emotions. This is mind.

When we look at experience, we find that both matter and mind happen in the same place. I am here, where my thoughts are. I am also here, where my body and physical things are.

If I am this body, and I live in the physical world, then how do I experience thoughts and feelings? I must not be a body. I must have a body, but I must be something that comes before the body.

Descartes’ paradigm of “res externa” (matter) vs “res cogitans” (mind) laid the foundation for the dualism that permeates the West. It is part of the waters we swim in. Becoming familiar with non-dual traditions cuts through this construct. One sees dualism as a mental framing, not as a reality in itself. This is quite a contrarian belief.