I Can remember the deep sense of mystery that possessed me when as a child I was told that only about one-eighth of an iceberg appears above water. It was perhaps one's first contact with the concept of the ‘assessable unknown’. In later years, in experiments in displacement and specific gravity in the physics laboratory, one learned how the conclusion had been arrived at—learned, indeed, how to assess the unknown for oneself. But increasingly one became aware that the process had its limitations: that while there were certain ‘unknowns’ which could be so thoroughly or so accurately assessed that they scarcely continued to merit the title ‘unknown’, there were others where the mystery remained. In some cases the assumption was justified that one's knowledge would increase as one grew older: or, on a more general world-view, it was warrantable to suppose that the sum total of human knowledge of any particular phenomenon would increase with the passing of time and the continuance of human research, until the day arrived when the phenomenon in question became, by general consensus, more ‘known’ than ‘unknown’.