A teacher must remain mentally alert. He will not get into the profession if of low intelligence, but it is all too easy, even for people of above-average intelligence, to stagnate intellectually and that means to deteriorate intellectually. A teacher must be quick to adapt himself to any situation, however improbable (they happen!) and able to improvise, if necessary at less than a moment's notice. (Here I should stress that I use 'he' and 'his' throughout the book simply as a matter of convention and convenience.)