These nine books, with the forty-five previously written, make in all
fifty-four.
Some were composed during the youth of the author, others when in
his bloom, and finally the last, when his body was already seriously
weakened; and they betray his condition while writing them. The
twenty-one first books seem to indicate a spirit which does not yet
possess all its vigor and firmness. Those that he wrote during the
middle of his life, show that his genius was then in its full form.
These twenty-four books may be considered to be perfect, with the
exception of a few passages. The last nine are less powerful than the
others; and of these nine, the last four are the weakest.