Philosophy of Ghost in the Shell
What exactly is the definition of 'human' in a society where a mind can be copied and the body replaced with a synthetic form?
What exactly is the 'ghost' —the human soul— in the cybernetic body, or 'shell'?
Where is the boundary between human and machine when the differences between the two become more philosophical than physical?
An inability of consciousness to distinguish reality from a simulation of reality.
In Ghost in the Shell, the word "ghost" is colloquial slang for an individual's consciousness. In the manga's futuristic society, science has redefined the ghost as the thing that differentiates a human being from a biological robot. Regardless of how much biological material is replaced with electronic or mechanical substitutes, as long as individuals retain their ghost, they retain their humanity and individuality.
The concept of the ghost was borrowed by Masamune Shirow from an essay on structuralism, The Ghost in the Machine, by Arthur Koestler. The title itself was originally used by an English philosopher, Gilbert Ryle, to mock the paradox of conventional Cartesian dualism and dualism in general. Koestler, like Ryle, denies Cartesian dualism and locates the origin of human mind in the physical condition of the brain. He argues that the human brain has grown and built upon earlier, more primitive brain structures, the "ghost in the machine", which at times overpower higher logical functions, and are responsible for hate, anger and other such destructive impulses. Shirow denies dualism similarly in his work, but defines the "ghost" more broadly, not only as a physical trait, but as a phase or phenomenon that appears in a system at a certain level of complexity. The brain itself is only part of the whole neural network; if, for example, an organ is removed from a body, the autonomic nerve of the organ and consequently its "ghost" will vanish unless the stimulus of the existence of the organ is perfectly re-produced by a mechanical substitution. This can be compared, by analogy, to a person born with innate deafness being unable to understand the concept of "hearing" unless taught.
Ghost-dubbing, or duplicating a ghost, is a near-impossible act in the Ghost in the Shell universe. When performed, as a cheap AI substitute in Innocence and earlier in the manga, the result is always inferior to the original-which always dies in the process. In Stand Alone Complex, criminals use a ghost-dubbing device to create numerous duplicates of South American drug lord Marcelo Jarti; after the original died, the device continued to duplicate him into a near-infinite number of bodies with identical memories and personalities, essentially immortalizing him.
In Ghost in the Shell, Kusanagi completely reproduces the stimulus of all of her organs in order to maintain her "ghost". If a technical error arises during the transfer of a "ghost" from one body to another, the transfer normally results in failure, since the "ghost" tends to deteriorate due to either the difference of system at the material level or the deficiency of the transferring protocol. The Puppet Master manages not to deteriorate its "ghost" when merging with Kusanagi because his system is the body of information itself, thereby avoiding a deterioration due to the deficiency at material level.
The Ancient Greeks had a similar paradox, called the Ship of Theseus. Hegel's concept of Geist may also be related.