Although I’m not sure one can say with absolute certainty that anesthesia completely isolates the mind from external stimuli, you’re probably right. You’ve brought Helen Keller’s testimony to mind:

“Before my teacher came to me, I did not know that I am. I lived in a world that was a no-world. I cannot hope to describe adequately that unconscious, yet conscious time of nothingness. I did not know that I knew aught, or that I lived or acted or desired. I had neither will nor intellect. I was carried along to objects and acts by a certain blind natural impetus. I had a mind which caused me to feel anger, satisfaction, desire. These two facts led those about me to suppose that I willed and thought. I can remember all this, not because I knew that it was so, but because I have tactual memory. It enables me to remember that I never contracted my forehead in the act of thinking. I never viewed anything beforehand or chose it. I also recall tactually the fact that never in a start of the body or a heart-beat did I feel that I loved or cared for anything. My inner life, then, was a blank without past, present, or future, without hope or anticipation, without wonder or joy or faith.”

Yes, I have to admit that I don’t have any example of consciousness in an individual completely without external stimuli. In fact, I agree with you on the last point you made. My point is that we need to be more precise about what we mean by “external stimuli” exactly.

We could even say that it may be impossible to isolate the mind completely from all stimulation in the first place. That leads to the real issue: perhaps those external stimuli are always present in some form, even if they are extremely minimal, indirect, or invisible to us at first glance.

So I correct myself, the argument is not necessarily that consciousness can exist with literally no input whatsoever, but rather that what we call “external stimuli” may include things so subtle that they are easy to overlook. In that sense, the claim becomes less about the absence of stimulation and more about how little stimulation is actually enough to sustain consciousness or mental activity.

You’re mixing up two different things. One is normal brain development, and the other is consciousness itself. It’s true that a human being needs certain external stimuli for the brain to develop in a healthy way and for mental experience to be normal and stable. But that does not mean that, without those stimuli, consciousness does not exist.

A person can still be conscious and yet suffer severe cognitive, emotional, or behavioral changes because of sensory deprivation or an extremely limited environment. What changes is the quality of development, mental functioning, and maybe the kind of consciousness they have, not necessarily the existence of consciousness itself.

Also, there is no clear way to measure how much, how little, or whether someone has “no” consciousness at all. Because of that, claiming that a lack of stimuli means a lack of consciousness goes beyond what can actually be established.

So your argument seems to confuse “without stimuli, there is no normal development” with “without stimuli, there is no consciousness.” Those are not the same thing.