I’ve personally seen Montessori principles work again and again at the Montessori School that I own; and, more recently, I’ve been able to apply the same principles to help adults to live happier, more succesful, more meaningful lives.
The principles that Maria Montessori discovered also work for adults because they are universal principles of human nature–principles such as normalization, deviations, freedom, prepared environment, observation, and experimentation.