The first thing the student should bear in mind when starting to investigate any subject is: everything that has already been affirmed, has already been refuted; everything that has already been praised, has already been criticized; everything that has already been taken as a rule, has already been violated; and for every example of a given theory, a given current, a given style or a given inclination, it is possible to find a contrary example. This is fundamental for the student to be cautious and never open a book expecting an end point. Good learning is stimulating, it encourages more study and not less, it moves rather than paralyzes.