Wing.

Wings, like hands, feet, or ears, are perceived by us as a pair. Inexistent without each other. 
I thought exactly the same until I read the beautiful parable of the wing. I won't quote it here in the description. I put this parable right on the perimeter picture. And every one of you can read it if you want to.
I would like to dedicate this painting to all handicapped people. All those who, in spite of their physical condition, remain active and useful members of society.
I know how hard it is for you. I am proud of you.
Let this wing sound metaphorical, as a symbol of willpower.