I have a theory. It is a bit complicated, but here it is [Actually, now that I re-read it, the whole thing seems like a horrible mess. I will try to come back and streamline it later.]
I start with a distinction between Being and existence. I use the term ‘existence’ to refer to determinate things — which are basically things that can be experienced as “being the things that they are” and are thus experientially distinguishable from the things that they are not. So, for example, I can experience a cat as a cat in the sense that I can distinguish it from a dog or a door knob. Sensations and mental states also exist, since I can distinguish my thought “2+2+4” from my thought that “2+3=5”, and so on. Basically any object of experience that obeys the logical “law of excluded middle” counts as an existing thing.
With the term ‘Being’ I want to get at a broad concept that includes both existence and nothingness. Those of you who understand Martin Heidegger will have some idea of what I’m getting at here. Heidegger was influenced by Eastern philosophers who talk about “nothingness” or “emptiness” or the “Void”. The basic idea is that nothingness is an indeterminate mode of Being. It is not mere nihilistic, absolute nothingness, but rather, it is a sort of pure potential. I compare it, metaphorically, to white light because white light can be thought of as being no particular color at all, but as the potential for all colors. So Being consists of at least two basic modes: Determinate (which, in my metaphor, is like a specific color of light) and Indeterminate (“pure white light”). These metaphors are weak in several ways, but they are the best I can do at the moment.
I also use ‘experience’ as a broader term than ‘consciousness’, so I can talk about both conscious and non-conscious experience. Experience in the broad sense is like “white light” — it is an indeterminate mode of Being. Consciousness is what-it-is-like when the “pure possibilities” of indeterminate experience organize into determinate processes (like filtering white light so that specific colors emerge). Conscious experience is always “consciousness of” some object or other because it is determinate in the sense that one can always, at least in principle, distinguish the phenomenal qualities (aka “qualia”) of an given conscious experience from the qualia of any other conscious experience. What gives consciousness the specific feel that it has at any given moment is the fact that it is a moment of Being (thus fundamentally experiential) that has taken determinate form (there is something specific that it is like to be…). You might say that Being is “composed” of potential moments of “what-it’s-like-to-be”, and existence is the manifestation of these potentials in determinate moments (like a prism filtering specific colors). Consciousness is Being experiencing Itself in this determinate mode.
On the basis of these distinctions, I would say that EXPERIENCE is not necessarily tied to physical processes, but CONSCIOUS experience IS necessarily tied to physical processes. The reason that conscious experience must be tied to physical processes can be summed up this way: What makes conscious experience CONSCIOUS is the fact that determinate qualia are experienced, and what makes qualia determinate is the implicit capacity to distinguish one quale from another. It feels different to think of the number 2 than it does to think of the color red. Conscious experience is necessarily limited to a more or less specific focus. A quale is what it is because it can be distinguished from everything that it is not, and because of the nature of the way in which it is related to everything else in the web of possible determinate experiences. To be physical is to be a determinate process, in other words, to be “Being experiencing Itself” in a determinate way. A physical system is “physical” precisely because it is determinate, which is to say, it is Being experiencing Itself from a certain perspective. (This “perspective” is what determines exactly which determinate experiences emerge from the indeterminate background. It is the “filter” that specifies which colors emerge from the white light.) To experience the visual sensation of seeing an apple, there must be some physical system that is capable of distinguishing “this particular apple” experience from all other experiences. This does not necessarily have to be a BIOLIGICAL system, but it does have to be a physical system.
But we need to note at least one more thing. Not all discriminating physical systems are conscious systems. I don’t consider a thermostat to be a conscious system, even though it is a physical system that can distinguish hot from cold. Why isn’t a thermostat conscious? This topic is too big to discuss here, but I think SELF-ORGANIZATION is the key. The manner by which a thermostat distinguishes hot from cold involves a simple algorithm. The thermostat’s processing is a matter of “top-down” control. A fancy computer programmed to mimic human behavior would be the same way. To be capable of conscious experience, a machine would have to be designed so that the means by which it makes discriminations emerges from self-organizing components. This is how biological brains work, and this is how machines will have to work if the are to be truly conscious. There is a deeper reason why self-organization is key, but it’s too much to discuss here. I have addressed self-organization in some of my other answers.
Summary: Conscious experience is possible because Being has both a determinate and indeterminate mode. The indeterminate mode (aka, the mystical “Void”) is the metaphysical “raw material” out of which determinate modes of Being (aka “physical processes”) can emerge. Indeterminate Being is experiential, but not conscious. Conscious experience emerges from the self-organization of determinate processes that are capable of discriminating one thing from another. Since “being physical” basically means “being a determinate process”, and since only determinate processes can distinguish one determinate thing from another, it follows that only physical beings can have conscious experiences. The notion of “indeterminate experience” may be incomprehensible, but I am betting on the enlightened sages here, who assure us that experiences of this sort are possible, although not comprehensible by rational cognition.
Of course if my theory is wrong, or if the sages are just kooks, then all bets are off 😉