The following is an extract from Frankenstein by Mary Shelley. Is the "had not been" correct? Should it have been "would not have been"?
"A human being in perfection ought always to preserve a calm and peaceful mind, and never to allow passion to a transitory desire to disturb his tranquility. I do not think that the pursuit of knowledge is an exception to this rule If the study to which you apply yourself has a tendency to weaken your affections, and to destroy your taste for those simply pleasures in which no alloy can possibly mix, then that study is certainly unlawful, that is to say, not befitting the human mind. If this rule were always observed; if no man allowed any pursuit whatsoever to interfere with the tranquility of his domestic affections, Greece had not been enslaved; Caesar would have spared his country; America would have been discovered more gradually; and the empires of Mexico and Peru had not been destroyed."