Here is Avicenna on the suspended man:
We shall say, therefore, that someone from among us ought to be thought of as if he were created all at once and full grown, but with his eyes covered so that he would not see external things. And he would be so created as if he were moving in the air - or in a void, in such a way that the density of the air would not touch him that he might sense it. And his limbs would be, as it were, spread out in such a way that they would not come together or touch one another. Now let him see if he affirms the being of his essence. For he will have no doubt about affirming that he exists. Yet he will not affirm outward things about his limbs, or interior things about what is inside him, neither his mind nor his brain, nor anything else outside him. But he, whose length or breadth or depth he will not affirm, will affirm that he exists. If, however, it were possible for him at that time to imagine a hand or another limb, still he would not imagine it to be a part of him, or necessary to his essence. Now you know that what is affirmed is other than what is not affirmed, and what is granted is other than what is not granted. And, because the essence that he affirms to exist is proper to him, insofar as he is that very essence, and is something besides his body and his limbs, which he does not affirm, therefore, once he has been awakened, he has a pathway to proceed in full wakefulness to knowing that the being of his soul is other than the being of his body. Indeed, he does not need the body in order to know the soul and perceive it. But if he is a dullard, he will have to turn to that way [and rely on the body to gain a knowledge of the soul].As you can see by reading the passage, the idea is to assume that an adult human is created "all at once" but suspended in a state of zero sensory stimulation, in fact, zero 'external sense' to use a term of Kant's. Also, his isolation will make it impossible for him to make assertions about his interior: "Yet he will not affirm outward things about his limbs, or interior things about what is inside him, neither his mind nor his brain, nor anything else outside him." Why can he not "affirm" these things? Simply because he has no input, no basis upon which to make claims. He is in a state, permanently, (and from the first moment of his life mind you), like those who use John Lilly's isolation tanks.
His is an utterly featureless world, yet Avicenna supposes that this person would be able to 'affirm' his own existence as a sort of Cartesian center of consciousness (even as he/she never has experience). One wonders about this. Are there contents of such a mind? Can a mere center of consciousness, which has never been aware of anything interior or exterior, be aware of itself? Doubtful, if not impossible. But, let's grant that this Cartesian point can be aware of itself, affirm its own existence. What does Avicenna say we can conclude from this as regards the metaphysical status of that consciousness?
First he makes what seems to be an elementary logical distinction. Anything one might affirm or claim is obviously going to be identical with itself, and NOT identical to anything that you do not claim. In affirming or claiming existence for oneself as a center of consciousness, one is not making a claim about anything else. Given, next, that this conscious entity is, by hypothesis, NOT making claims about arms legs or brains, indeed, not making claims about anything external to itself, nor claims about his/her 'innards,' so to speak, because he/she has no information from which to craft such claims or beliefs, we are left with a very simple situation. In fact, this person can make only ONE claim as to what he is. To use the bagage laden language of Avicenna; He is simply a 'soul,' a center of consciousness.
If, afterwards, this center of consciousness is presented with sensory data of an arm or a leg, it is not inevitable that it will occur to him that these things are his or part of him.
Therefore, Avicenna concludes, this center of consciousness can tell that "the being of his soul is other than the being of his body." In other words, he can conclude that he, as a consciousness, is something that does, and can exist independently of any material object.
But, does this last metaphysical conclusion really follow from what precedes it?
We have a move from this:
'He can claim only that he is a center of consciousness. He cannot claim anything else.'
'He is only a center of consciousness. He is not anything else.'
This is quite a leap, as this analogous argument shows: Suppose Fred is in a building, and oblivious to what is going on outside. Fred has no basis to claim that Barney is outside the building. Yet, assume Barney is out there, cooking bronto-burgers on his grill. So we have:
Fred can claim only that he is in the building. He cannot claim (because he has no evidence to that effect) that Barney is outside the building.
From which we certainly cannot infer:
Fred is in the building, and Barney is not outside the building.
Logically the latter does not follow from the truth of the former.
Ditto for the pair of statements from Avicenna. It does not follow from the fact that the suspended man makes no claims as to his own corporality, that he is not in fact dependent upon corporality. It may be true that he is (among other things) a center of consciousness, but it does not follow, either that he is aware of all aspects of his being, or that there are not other aspects of his being, amongst which are aspects more basic and supportive of the consciousness.