And by what means are we to know and measure this supposed truth? And against what?

As far as I know right now, who I am is something I am continually learning about and will probably forever remain a mystery, because I can ultimately only know myself from one perspective, from within. And there is so much to know and in so many situations. I think the answer to this question is very relative.

I mean, just ask several people about one person. You'll get several different description. Who is that person REALLY? I guess that depends on what you mean by REALLY.

Who is a person? Are they their memories? Their personality? What they do? Their consciousness? All these phenomena change over time and in varying circumstances. Just because the mind can and prefers to generalize ... this does not mean the generalizations are valid. There are many that propose that the notion of "self" is merely a convenient, functional illusion generated by language ... that is, the way our language makes us tend to generalize about experience. Those people might be right. I just don't know.