There are three principal means of acquiring knowledge... observation of nature, reflection, and experimentation. Observation collects facts; reflection combines them; experimentation verifies the result of that combination. Denis Diderot
There's nothing more important than our good health - that's our principal capital asset. Arlen Specter
Our life is a book that writes itself and whose principal themes sometimes escape us. We are like characters in a novel who do not always understand what the author wants of them. Julien Green
Love is something far more than desire for sexual intercourse; it is the principal means of escape from the loneliness which afflicts most men and women throughout the greater part of their lives. Bertrand Russell
All animals, except man, know that the principal business of life is to enjoy it. Samuel Butler