If you are flying in a propeller airplane from west to east and a cross wind is blowing from the north you have to point the plane in a northerly direction to get to where you are going.
If you point the plane at the exact destination you are heading for the north wind will cause the plane to land south of the direction you are aiming for.
Mankind is like the airplane: if we treat a child, a man, a woman as if they are normal, they will drift southward and become less than they could be.
If you treat a child, a man, a woman expecting them to be better than they can be they will become what they could be.
Who first suggested this? Johann Wolfgang von Goethe:
When we treat man as he is we make him worse than he is.
When we treat him as if he already was what he potentially could be
we make him what he should be.