"the same" versus "sameness"

(Grammar Questions I Couldn't Answer)

My wife (L1 Vietnamese) was reading a novel in English, and came upon the word "sameness" being used in a sentence.

"Is this a real English word?" she asked.
I said yes.  (It was low-frequency, certainly, and maybe slightly unusual, but it still seemed to fall within the bounds of acceptable usage.)
"What does  it mean?" she asked.
"It's just the noun form of 'same'," I said.  "You know how '-ness' can change an adjective into a noun--like 'happy' and 'happiness'."
"But isn't 'same' already a noun?" she asked.
"No, it's an adjective.  For example: 'That is the same book'."
"But how come we use it with 'the'," she said.  "We often say 'the same'.  I thought 'the' was only used in front of nouns."

...and this I couldn't answer.

Update:

I looked this up in  Practical English Usage by Michael Swan.  Swan said:
"We normally use the before same. Give me the same again, please." (p.490).  But Swan never says why.

Update, Update:

I asked my manager about this, and he said the reason we use "the" before "same" is because there's always an implied ellipted noun phrase.  So "Give me the same again"  is short for "Give me the same thing that I had before again."