It's been inspired by Roger Evans' recently published "5DL - Five Dimensions of Leadership", available from Creative Leadership Consultants.
The actual piece that Roger describes this one-liner in is on page 93 in the chapter "1DL: The Ability to Self-Reflect"
"So as we consciously stop and reflect so we begin to realise that we are actually the subject of our thoughts and feelings rather than being the victim or object of ‘what’s out there’. This is for many such a powerful insight!
Every sentence, thought, image, piece of music, feeling, etc that occurs to you or appears to you in your mind's eye - it is all about you. Pay attention, grow your self-awareness, take control over your mindsets and from there control your life!