We cannot yet define consciousness without tautology or hand-waving, but we have learned enough to devise a few supportable statements about it:
1. it happens inside the brain
2. more than one neuron is involved at any given point
So, it's in one place (the brain), but it's not in one place (many neurons) ...
The unitaryness of consciousness is an illusion. That my consciousness feels singular is merely a manageable fiction. The prefrontal cortex demands it thus. The neocortex lives by the assumption that “me” is one thing. Planning cannot exist without identity. Identity cannot exist without disambiguation — the ability to discern that which is me, from that which is not me. One hundred billion neurons in polyphonic chaos all agree. They instinctively collude to lie to their corporeal host saying, “Yes, you exist, and yes, you are one.” Sorry, Descartes.
Consider the post-operative corpus-callosotomic, a patient whose brain hemispheres were surgically separated. The two hemispheres of the brain being severed, two brains now exist within a single corpus humanus. One side of the brain will make attempts to answer for the actions of the other, but these attempts will be feeble and inaccurate (Really interesting experiments showing pictures to one eye only and asking questions). Consider the cortically blind (due to lesions in the striate cortex), who respond to visual stimuli that they do not consciously see. Without "seeing", they avoid obstacles and step up on curbs. It's almost like there's a separate little decision maker living in their brains.
As a human, I share more with octopus consciousness than I’d prefer to believe. The neurons of a highly evolved cephalopod are not concentrated in the brain like ours. They do have brains, but two thirds of their neurons are in the arms. Complex motor skills are not organized in their brain using an internal somatotopic map of the body. Instead, octopus arms show a variety of complex behaviors even in the absence of input from the brain.
My consciousness emerges continuously from one hundred billion separate neurons. Why couldn’t octopus consciousness emerge from its separated parts?It takes a human to look at such a nervous system, and call it “distributed." We say it pejoratively. We condescendingly marvel at how it works, as if the human one were any different.
Octopi can be trained to distinguish different shapes and patterns. They practice observational learning. They discover, on their own, how to open a jar, and they remember how. Octopi play. In the UK, since 1986, the common octopus (Octopus vulgaris), is protected under the Animals (Scientific Procedures) Act. One cannot operate on them without anesthesia. In 2013, this legislation was extended to include all cephalopods.
My consciousness exists (or seems to exist) within an ecology of energy patterns that have bearing on the outside world (aka meaning) only via organized relationships with sensory and motor neurons. It’s all about context, isn’t it, Wittgenstein?
If we keep developing the true statements we make about consciousness, we'll eventually be able to define it.