It is this kind of knowledge that the true teacher needs to know. This does not absolutely mean that no one can teach with fullness of knowledge, nor does it mean that everyone who knows his lessons thoroughly will teach successfully. But imperfect knowing makes imperfect teaching. What a person does not know he cannot teach, or, if he teaches it, he cannot know that he teaches. However, this is but one of the laws of teaching.
Success comes from the obedience of other laws as well.
Failure comes from violating them. The power of illustration is the chief and central power in the teacher s art. It comes only out of clear and familiar knowledge. The unknowing teacher is like the blind trying to lead the blind with only an empty lamp to light the way. To such teachers geography is but a chapter in the science and history of the universe, borrowing meaning from all that goes before or follows.