Theory number 3.

[The Grand Nudger]

Again, that's precisely the problem, there is no accepted operational definition, there is no cutoff...but that doesn't mean we can't impose one for discussion, experiment, or just to break it up into smaller chunks to make it easier to communicate or parse (so long as we're willing to acknowledge that the line we have created is ultimately arbitrary, circular, or fuzzy -perhaps all of the above-). All qualifiers would somehow involve wealth or depth of experience, awareness. That this awareness or experience is itself so far undetectable (Apo and I might suggest that this is because it is not actually -there-, as we commonly think of it, more of a shorthand term for an aggregate process) leaves us with nothing beyond effect and architecture to determine whether or not any give example meets our (likely arbitrary, circular, and fuzzy - see above) definition of consciousness. One of the first criticisms of any definition we give will be a reference to the demonstrable fact that two unrelated structures, dissimilar in organization, can achieve a similar or identical effect. So, if we say the cutoff is neurons or central nervous systems (decidedly higher order architecture) we have to at least acknowledge that fundamental effects we propose as "consciousness" or at least evidence of it ca be achieved by something like a thermostat...which we do not intuitively feel is "conscious" (though simultaneously a central nervous system can achieve much more than a thermostat).