The early music movement, born in the 1950s, focused on restoring period instruments and reviving authentic performance practices. However, the question of authenticity largely neglected the harmonic thinking of composers, particularly Bach's, as described in contemporary sources. The analysis of the B minor fugue from the first part of the Well-Tempered Clavier by Johann Abraham Peter Schulz, written at the request of his teacher Johann Philipp Kirnberger, is a telling example of this gap. This analysis, the only known example of a complete fugue analysis by Bach's contemporaries, reveals that Bach's music is built on structured progressions of Grundbässen, which I call "mental bass." My edition presents the harmonic solutions to 24 Preludes and Fugues from the Well-Tempered Clavier, using the mental bass corresponding to Schulz/Kirnberger's Grundbässen, offering a new perspective on Bach's harmonic thinking.