"The Ghost in the Machine" is a metaphor for the mind-body problem, particularly within Cartesian Dualism. In other words, if there are bodies and souls, how does the soul control the body? Since the physical world seems to be causally complete (i.e., everything has a cause), it doesn't appear a soul has any room to effect anything. The solution shown is Leibniz's Pre-established harmony principle, which said that the mental and the physical only coincide by a kind of coincidence, because God set it up that way. This solution is obviously not very easy to take seriously, in particular for modern readers. On a side note, Decartes's own solution was even worse, he claimed they interfaced with a special organ within the body which acted as a kind of antenna: the pineal gland (the true function of it was unknown at the time).