I'd like to share a simple thought experiment that has some pretty strange implications. It seems to show that the world cannot always be sharply divided into separate experiences.

We can make the analogy, to how the world cannot always be divided into separate organisms. Usually it can, but there are cases, like conjoined twins, where someone would say it's one organism, someone else that they are two organisms, and someone else that it's something in between.

The experiment lies on two postulates:

P1. Mind-body continuity

It's the idea that tiny physical changes should correspond to tiny mental changes. And that should work for arbitrarily tiny ones.^[1] So you couldn't completely change the content of your experience, just by moving one atom in one of your neurons.

An infinitesimal physical change, will cause an infinitesimal change of that system's behavior. So if you rejected mind-body continuity, you'd have to admit that some non-infinitesimal mental change can have an infinitesimal (practically zero) effect on the physical world. So basically you'd be forced into near epiphenomenalism.

P2. Splitting the brain in half creates two separate experiences

This is illustrated nicely by the experiments with split brain patients (like this one). For example, if you had a split brain and closed your eyes, you couldn't tell if your right hand and left hand are touching the same surface. Those two experiences are separate^[2] - they aren't unified into one, so you cannot compare them.

(It’s not relevant here if they should be treated as two persons. I only claim that they are two experiences.)

Experiment

Imagine that you undergo a brain-splitting surgery. You are anesthetized so you don't feel pain, but in this experiment, you're fully awake. And this surgery is very slow - they cut only one nerve fiber at a time.

At the beginning, you have one unified experience, but at the end you have two. So one possibility is that there was some exact point when the split occurred - some critical fiber that needed to be cut. But that would violate the mind-body continuity that we assumed. So the only alternative is that we gradually went from one to two experiences. That there was some "in-between" state.

What does it feel like?

So, how would it feel to be in such a state? Here is a real-life example of brain-conjoined twins where we may say that such "partially connected" experiencing happens. The blindfolded girl can access what her sister is seeing, but it's faint so she needs to focus.

Maybe it's just about how tightly bound the qualia are? So qualia of one sister are bound to the second one's, but very weakly. If that's true, this would make it much less exotic - in your ordinary mind you also have some qualia which are tightly bound, and some which are almost disconnected. For example, when you look at a tree:

• the greenness of a leaf if strongly bound to the leaf's shape
• all the leaves and the tree body are bound into one object, but it's a weaker binding
• the tree and the sky and the grass are all bound into a unified visual field, but it's a weaker binding
• your visual and auditory fields are bound into your experience of the external world, but it's a weaker binding
• that external world is bound with your thoughts and emotions and body perception to form your whole unified experience, but it's a weaker binding

Speculation

When you stop paying attention f.e. to your auditory field, could it be that it becomes disconnected from the main chunk of your experience (visual+thoughts+...), but those sound qualia maintain some "smaller" existence of their own?

Or that the dorsal visual stream (the one that is responsible for spatial understanding, and is separate from the object recognition stream) has its own "smaller" experiences?^[3] Unfortunately, this stream is disconnected from your thoughts and episodic memory, so we don't have any easy way of checking that.

If we could identify some brain systems that are usually disconnected, but sometimes do connect, we could use @algekalipso's phenomenal puzzle test, to see if the other system has its own phenomenal experience.^[4] But it may be impossible to find systems suitable for all the steps in that test.

Consequences

Integrated Information Theory, which is currently the most formally developed theory of consciousness, explicitly aims to divide the world into separate non-overlapping experiences (see Exclusion Postulate). The math used in that theory allows for situations, where an infinitesimal physical change results in a drastic mental change. It violates the "mind-body continuity"^[5]. 

So maybe that Exclusion Postulate should just be dropped? The rest of the theory could still work without it. Instead of a set of separate experiences, the theory would output some more complex structure, maybe a graph^[6]. For practical purposes, we could still cluster this graph into a set of separate experiences like before, but it should be clear that this is not what really exists. It’s just a practical simplification. And we could still quantify the amount of experience with a suitable graph metric.

So far, this mind fuzziness has never been a noticeable problem in real life. But brain-computer interfaces, mindmelding and digitalization of minds, will enable much more exotic mental architectures. This also complicates utilitarian calculations. But we can still aim for "the greatest good for the greatest number" - it's just that this number is no longer an integer.

Thanks to Kamil Przespolewski for all the help!

1. ^{^}

   This postulate can be formalized analogously to Weierstrass's definition of a continuous function. So for an arbitrarily small mental change magnitude, you could find a physical change that produces a mental change with even smaller magnitude.

2. ^{^}

   It is disputed if a complete disconnection happens in split-brain patients. Some information transfer may still be possible through the brainstem. But for our experiment it's enough to assume that this complete disconnection is possible in principle.

3. ^{^}

   This would mean that blindseers have some visual qualia too, but they just cannot report it. 

4. ^{^}

   It may not be possible to freeze one of those systems, as the test requires, but at least we could distract it, to a similar effect.

5. ^{^}

   Creators of IIT explicitly embrace this discontinuity. See endnote xii in their 2014 paper.  Another discontinuity scenario for IIT  is raised by Schwitzgebel here.

6. ^{^}

   I expect this graph to represent the binding relations between qualia in some way. (Maybe a hypergraph would be more suitable.)