Elisabeth, upon hearing of Descartes' failed attempt to converse with her, wrote to Descartes a letter. In this letter, dated May 16, 1643, Elisabeth writes, "tell me please how the soul of a human being (it being only a thinking substance) can determine the bodily spirits and so bring about voluntary actions". Elisabeth is questioning Descartes' idea of dualism and how the soul and the body could interact. Elisabeth rightly questioned how something immaterial (Descartes' idea of the mind) could move something material (the body). Descartes replied to Elisabeth's letter with the answer that this interaction should not be thought of as between two bodies and that it is the kind of union that exists between the two qualities of heaviness and bodies.