By what criteria do we judge as to whether we have understood consciousness on a theoretical basis?
One criteria might be the construction of AI (but this can't be the whole story as everyday new conscious's are born - ie birth). Still, one cannot experiment (ethically) on consciousness; and one notes how our understanding of how consciousness is constituted has been enlargened by pathologies of the mind or brain; but one can experiment (presumably ethically) on AIs; which should bring some additional insight.
The obvious criteria is that of the Turing Test; however, important as this test is, to a large extent it misses the main issue; what the Turing Test discovers is how well can one simulate an intelligence. Simulation isn't the thing itself, and though there is a level of disagreement/controversy about the notion of 'inner life' or qualia - to my mind its of the essence for this question.
The Turing Test puts in a sharp form the test for a construction of an AI.
The sharper question is how does one know one has consciousness-in-itself or a simulation?
What other criteria are possible, necessary or sufficient?
Or is the horizon of this criteria infinite (ie impossible).
To go back to human beings, we denote ourselves by consciousness, by the cogito, and to others, by some principle of continuity or resemblence (I am human, you are human; I am conscious, you are conscious); which can be put in syllogistic form; but the essence, surely of our recognition of consciousness in others is continuity, or even more basically, resemblence.