We hear music in two keys at once...
When we first learn that this thing called 'Music Theory' actually exists; our first encounters with it involve grouping designations of notes together and giving these groups of tones names -- such as: Scales, Chords, Triads, Arpeggios, etc...
At the college level; we learn to identify with more terms: Dominant, Sub-Dominant, Tonic, Cadences, Resting & Active Tones, etc... All of this rigorous study was designed to develop our ear... and was necessary to develop a vocabulary that musicians may use to communicate -verbally- what they hear.
After all is said and done in our 'formal study of harmony' (if we have studied well) --we may come to a fundamental understanding of the way the harmonic overtone series most naturally reacts to melody that is created using elemental tetrachords taken from our Major Scale.
For us - there is a difference between music 'theory' and music 'fact.' -- One example of a music fact would be something like: "There are three flatted notes in the 'Eb' Major Scale (according to the design of our equal-tempered 12-tone system)." --- Now, if this statement is taken as a fact, then - What is music theory?
We find the implications of the following statement to be in the order of the mystical - and would like to suggest that it represents the heart of true-music-theory... the kind of theory that requires both hemispheres of our brain (and a lot of experience listening and playing) to grasp:
'We begin in the tonal center of 'C' --- When we hear the fundamental tone 'Do' ('C') - the tone 'So' ('G') is heard, physically, as the "Dominant" overtone in the natural acoustics of the harmonic overtone series. This tone ('So') is so "loud" in relation to all other overtones being propagated in the natural harmonic overtone series that the ear 'may' hear all the tones of the ‘C’ Major Scale in relation to the tone 'So' ('G') – as if it were, itself, the “fundamental" (‘Do’). What this means is that the ear 'may' hear the tone 'C' (the true 'Do') as 'Fa' -- and the tone 'F' (the true 'Fa') as 'Te' (a flatted tone). This suggests that any flatted tone may - most naturally - be heard as 'Fa' of a new Major Scale --- and any 'Do' (a resting tone) may also be heard as 'Fa' (an active tone). It also goes without saying that any 'So' may be heard as 'Do.' This is why it can be said that the 'Blues' assumes that this 'Diatonic Pivot' had already occurred before the music began.'
We hear music in two keys at once... even though we focus our understanding in one key at a time.