Descartes uses the metaphor of a pilot and his ship to elaborate on the relationship between the soul and the body. According to what's called Descartes's substance dualism, the mind is distinct from the body. Yet at the same time, it's impossible to avoid the conclusion that some kind of relationship pertains between these two radically different substances. But how to account for this relationship? And how is it possible for there to be any kind of casual relationship between mind and body, given that causation as such only applies to objects in the spatio-temporal world—the world around us, the world in which he live, move, and have our being?
To show how it's possible for mind and body to interact, Descartes introduces the metaphor of the pilot and the ship. Just as a pilot steers a vessel, the soul controls the movements of the body. In steering a ship, the pilot makes it do certain things, such as navigating between rocks, for example. By the same token, the soul makes the body act in certain ways, such as the movement of limbs.
This is not to say that the mind and body are one; Descartes is a substance dualist—that is to say, he believes that there are two finite substances, mind and body—not a substance monist like Spinoza, who argued that mind and body are simply the same thing conceived under different attributes: respectively thought and extension.
Many scholars have expressed dissatisfaction with Descartes's metaphor, which they see as telling us what the relationship between mind and body is, rather than how it actually operates. A further bone of contention arises from the fact that, in the metaphor, both the pilot and the ship and objects in space and time, which means that causation can legitimately be applied to their interactions in a way that would not be possible between two radically different substances, such as the mind and body in substance dualism.