What is the Phantom of Truth?

"...she knew and read my thoughts as I read hers, for we had understood the Mystery of the Hyades and the Phantom of Truth was laid." So writes Jack Scott in the final pages of Robert W. Chambers' "The Yellow Sign." But what is meant by that expression? Does "Phantom of Truth" really refer to a spooky person or monster, like the phrase "Phantom of the Opera?" Or is the phrase more similar in its meaning to "the myth of the given" and "the illusion of depth?" Join me as I lay the matter bare!