In the beginning this was Self alone, in the shape of a person (purusha). He looking round saw nothing but his Self. He first said, 'This is I;' therefore he became I by name. Therefore even now, if a man is asked, he first says, 'This is I,' and then pronounces the other name which he may have. And because before (purva) all this, he (the Self) burnt down (ush) all evils, therefore he was a person (pur-usha). Verily he who knows this, burns down every one who tries to be before him.

In plain words: At first only the Self existed, in the shape of a person. He looked around and saw nothing else, so his first words were "This is I." That is why people still say "It's me" before giving their name. The text explains the word purusha (person) as the one who burned away all evil before anything else existed.