If someone were to look at you right now they would see you as relatively symmetrical--there may be small details here and there that are not completely symmetrical, but generally your arms and legs are the same length, the two sides of your torso look similar.
Now try this: lie on the floor, and imagine a plane dividing yourself into left and right sides. Compare your perception of the two sides. Notice the length of each side, whether it feels rounder or flatter, wider or narrower, lighter or heavier. You may find some remarkable asymmetries in your subjective perception--one leg may feel inches shorter than the other, or you may feel like one side of you is flat and the other floating off the floor. Our subjective perception is much more malleable than objective observation. And this malleability is a key tool of the Feldenkrais Method.
Our subjective perception of our self is much more than an imaginary construct. It is our self-image, the foundation from which we act and live. And our interactions with the world change our self-image. It shrinks and expands along with our changes in activity, mood, and awareness. If I am playing the flute and feel expansive and happy, I will sense my size and use my lungs, hands, and fingers in a completely different way than if I feel tentative or nervous. And the reverse is true--if I can feel expansive when I am tentative or nervous, chances are I will begin to feel happier and more confident.
Try lying on the floor again. Now notice how the back of yourself comes into contact with the floor. The floor is a solid, objective surface. Comparing the contact between the back of yourself and this surface joins your subjective and objective perception. Notice the differences between the contact of your right and left sides. Roll very slowly a little to the right, to increase the contact of the right side, and then a little to the left. Do this several times, feeling the changes in contact, how there is more pressure on one side, then the other. Then lie in the middle again, and sense your contact now. Does it feel different than before?
This is just the beginning of an Awareness through Movement lesson. The lesson would usually go on to explore a movement in detail with several variations, and use a scan like the one above to see how the movement explorations have shifted your perception of yourself. These lessons help you become aware of differences between subjective and objective worlds. There is much more to consider--the subjective/objective dichotomy is the subject of volumes of philosophy and cognitive science--but these immediate practical applications help us live more fully. I'll add more later this month.