^{page 154 note 2} Allen and Unwin, 1950.

^{page 158 note 1} I should, perhaps, record that one historian of my acquaintance admitted at once that, strictly speaking, there were no facts of history (except statements such as those in I to 4 above).

^{page 158 note 2} It only causes confusion to say: “The universe is full of facts that we don't know.” We are discussing human knowledge, and human knowledge is what human beings know-not what they don't know. Of course we can invent any number of assertions, some of which may later turn out to be verifiable (such as “There are trees on Mars”), but until they are verified, they are not known to be true, and consequently cannot be used in raising a structure of reliable human knowledge.