No matter how thoroughly a person may have learned the Greek alphabet, he will never be in a condition to repeat it backwards without further training.

On the basis of the familiar experience that that which is learned with difficulty is better retained, it would have been safe to prophesy such an effect from the greater number of repetitions.

Often, even after years, mental states once present in consciousness return to it with apparent spontaneity and without any act of the will; that is, they are reproduced involuntarily.

Hermann Ebbinghaus Quotes