  As for general claims about substances based upon observed particular matters of fact, these too can qualify as real knowledge. Admittedly, it is only probable that, when four of the five properties previously found co-existing together occur again, the fifth will be present too. Yet we can still form an abstract idea of gold, as a substance having all five properties, and call this general claim knowledge, on the grounds that “whatever have once had an union in Nature, may be united again” (E IV.iv.12, p. 568).