Thank you for the direction. These articles seem to be the ones I were looking for. (Plato's work and Diogenes' answer is worth further investigation)

Human beings may not be that special. This search of mine may be futile, it being a remnant of my believer times (and I being raised in Eurasian soils).

Even so, "what is a human being" seems to be the right question to further investigate. I guess this will immediately be useful (if not already is) at information security: "is perfect authentication possible?" (with protection against rubber hose technique, or clones etc.)