Writers don't always know what they mean - that's why they write. Their work stands in for them. On the page, the reader meets the authoritative, perfected self; in life, the writer is lumbered with the uncertain, imperfect one.
It is a mistake to believe that science consists in nothing but conclusively proved propositions, and it is unjust to demand that it should. It is a demand made by those who feel a craving for authority in some form to replace the religious catechism by something else, even a scientific one.