  Based on the invariance principle of differential equations a simple,
systematic, and rigorous feedback scheme with the variable feedback strength is
proposed to stabilize nonlinearly any chaotic systems without any prior
analytical knowledge of the systems. Especially the method may be used to
control near-nonhyperbolic chaotic systems, which although arising naturally
from models in astrophysics to those for neurobiology, all OGY-type methods
will fail to stabilize. The technique is successfully used to the famous
Hindmarsh-Rose model neuron and the R$\ddot{\textrm{o}}$ssler hyperchaos
system.
