There is a long philosophical history which is responsible for the development of the notion of ‘spirit’, which culminated in the rationalism of the seventeenth century. Here’s a short version.
First there was the epistemologically underdeveloped idea of the spirit held by primitive, religious cultures (these being animistic… not yet anthropomorphic). This the result of the evolution of the frontal cortex, an addition that gave man the capacity for self awareness. The strangeness of this new self awarness was responsible for man feeling as if he had freewill; his deliberate acts of volition gave him the impression that he was something other than a mechanical body operated by the laws of nature. Trees blew in the wind, water flowed, rocks rolled down hills, but man could ‘choose’ to do what he did, he could deliberate and plan, foresee and anticipate possible outcomes, and this was something other natural things could not do. Now man unwittingly created a new metaphysical division (which will later be analyzed by epistemologists and rationalists of all colors); man no longer thought of himself as a dumb mechanical force, but rather as a ‘subject’ that does action… the ‘doer’ and the ‘deed’ now being seperate things. Enter the pronoun; ‘I’ raise my arm, ‘I’ eat a doughnut, ect. Formally, before self awareness of this kind, human ancestors just plodded along without language, and therefore without language oriented thinking.
Fastforward thousands of years. More advanced human beings, now called ‘philosophers’, use their now extraordinarily complex language to further define and describe the category of the ‘subject’, the ‘me’. This notion then becomes thought of as a different substance than that which things in the world are composed of. The greeks used an analogy of an instrument to describe how the subject relates to and is connected to the body. Is the spirit the instrument, the tuning of the instrument, the sound of the instrument, what? Does the spirit leave the body in the same way that an instrument becomes silent when it isn’t played. So on and so forth.
Now that the pronoun ‘me’ is a permanent fixture in philosophical language, philosophers from the dark ages forward begin messing around with it epistemologically. The father of a new and thoroughly thought out rationalism appears… Descartes, who’s famous dualistic system asserted that there were two distinct substances… one was material substance, the other spiritual substance, and the two interacted casually (later Spinoza would refute this dualistic theory). Kant comes in and calls this phenomenological ‘self’ a transcendent thing, whatever that means.
Summary: dumb human ancestors walk around unaware. Brain evolves. A neat little glitch in the processing centers creates the illusion of freewill (see ‘five millisecond rule’). Man becomes aware of his being aware and calls himself ‘me’. He says ‘I’ do this. Not only that this is done, but there is also a ‘doer’ who does it, and that is me.
Philosophers incorporate this erroneous concept of the ‘self’ into metaphysical religious theory. The old animistic systems are outdated and man begins to anthropomorphize metaphysics; there are intelligent human like gods up there and they created us, and unlike the imperfect, changing empirical world of things, man’s ‘spirit’, this second substance, is pure and immutable. It does not die with the body but goes somewhere else.
Fastforward to the age of positivism… where we are at the point of the p-zombie. Philosophers now argue that the way philosophers of the past treated the concept of the ‘self’ is erroneous… they talked about it as if it were a ‘thing’ (thanks Descartes) and applied to it what Ryle called a category mistake. You’ll have to google that because I ain’t about to explain it.
Now we arrive at Wittgensteins beetle in a box (google again). Previously the cartesian model of the ‘self’ proposed that the self was a private entity, something hidden from view, something in others we could not make contact with. Chalmers adds a neat little twist; if there was a private self in that brain, you couldn’t ever find it, because all we can observe is the behavior of others… not their mind. He tries to get around this problem with talk of qualia or Nagel’s ‘the eye that sees but cannot be seen’. There are problems with that too but we won’t go into it here.
Now my point earlier was to emphasize this problem of the private self and the fact that if you were to interact with a robot that could mimic human behavior, you wouldn’t be able to tell the difference between the robot and a real person (you’ll need to check out Searle’s chinese room experiment, too). The cartesian second substance of the spirit, there inside the body, is therefore a null question.
The behaviorists then come along and claim that the argument about ‘mental’ states is not the point. The point is that they are superfluous when only the behavior of human beings is in question. What they are ‘thinking’ is irrelevant, how they act and behave is the only thing accessible to the sciences.
Leave the leftovers for the philosophers chasing their own tails.