Dr. Karp applied common sense to the "problem" of crying babies that most new parents face, by borrowing from traditions being used around the world to calm babies down as if you had "flipped a switch." He acknowledges the existence of a 'fourth trimester', i.e. the first three months of the baby's life and the need to use techniques to make the baby feel as in the mother's womb to calm him/her down, appealing to their calm reflex.