Descartes himself struggled to come up with a feasible answer to this problem. In his letter to Elisabeth of Bohemia, Princess Palatine, he suggested that spirits interacted with the body through the pineal gland, a small gland in the centre of the brain, between the two hemispheres. The term "Cartesian dualism" is also often associated with this more specific notion of causal interaction through the pineal gland. However, this explanation was not satisfactory: "how" can an immaterial mind interact with the physical pineal gland? Because Descartes' was such a difficult theory to defend, some of his disciples, such as Arnold Geulincx and Nicolas Malebranche, proposed a different explanation: That all mind–body interactions required the direct intervention of God. According to these philosophers, the appropriate states of mind and body were only the "occasions" for such intervention, not real causes. These occasionalists maintained the strong thesis that all causation was directly dependent on God, instead of holding that all causation was natural except for that between mind and body.