In the Meditations, Descartes tries to prove that the human mind or soul is an immaterial substance distinct from the human body. Princess Elisabeth of Bohemia wrote to Descartes with an objection that many later readers are sympathetic to. If mind and body are two distinct substances with nothing in common, how can they affect each other? We all know by our own experience that mind and body do interact. So if Descartes' theory makes interaction impossible, then his theory must be false.