René Descartes wrote a philosophical essay titled "Six Metaphysical Meditations". Descartes thinks about the nature of reality, knowledge, and the human mind in six meditations that make up the book. Descartes employs a process of doubt in the meditations to methodically doubt all he knows and thinks in order to arrive at certain and undeniable truths. He has said that the only thing in which he is certain is his own existence, which is summed up in the idiom "Cogito, ergo sum" or "I think, therefore I am." On top of this basis, Descartes goes on to investigate the nature of reality, positing that there are two different kinds of substance: material substance (which encompasses the physical world) and immaterial substance. He also looks at how the mind and body are related, claiming that although they are separate, they do interact.