As we turn to consider in more detail how we may speak to men of the twentieth century, we must emphasize first of all that we cannot apply mechanical rules. We, of all people, should realize this, for as Christians we believe that personality really does exist and is important. We can lay down some general principles, but there can be no automatic application. If we are truly personal, as created by God, then each individual will differ from everyone else. Therefore each man must be dealt with as an individual, not as a case or a statistic or machine. If we would work with these people, we cannot mechanically apply the things of which we have been speaking in this book. We must look to the Lord in prayer, and to the work of the Holy Spirit, for effective use of these things.
From page 175 + 176., Volume One. The Complete Works of Francis Schaeffer ~ A Christian Worldviews.
(HT: Apologetics.com)
Human Beings Matter More 
Same goes with parenting books 😉