Paul’s Column anticipated to appear in this issue did not reach the columnist’s satisfaction. A few weeks before his sudden death, Paul De Bièvre decided to put the draft aside, until “more clarity on the matter” was gained.

Clarity in thinking and writing was Paul’s great theme as obvious from his Editorials and Columns in ACQUAL over more than 20 years. They were tutorial and instructive and an invitation to join him in exploring the “hidden structures behind the things we observe”—one of his favorite objects of investigation mentioned again in the last of his Columns. At such expeditions, the International Vocabulary of Metrology (VIM) provided him with reference points because “definitions of metrological concepts are lighthouses”. Consequently, Paul’s Columns often focused on concepts of specific relevance for metrology in chemistry, clarifying their structure and implications. He was particularly interested in ‘metrological traceability' and its related concepts, not least as it helps to avoid “circular reasoning”. Paul warned about such confusion once and again, and he suspected that, for instance, in current explanations and teaching of the base quantity amount of substance and its unit mole. The clarity of these definitions was his concern in numerous activities, endeavors and, of course, Columns.

Usually, Paul started writing a column with a clear message in mind. On the way, he found more and more facets important enough to be included—however, in the final approach, many of these valuables had to be sacrificed. Consequently, most Columns took their time, each half-sentence being carefully considered forth and back. Accompanying this process could be the editor’s challenge and pleasure alike.

Paul’s Columns are well observed, as indicated by frequent citations, responses and discussions. This may be exemplified by the two Discussion Forum contributions in this issue of ACQUAL. One of these refers to Paul’s Column published in October last year demanding clarity about amount of substance. The other contribution comments on his February Column, where once again Paul turned to the evaluation of proficiency tests: a dispute between ideal metrology and its practical implementation?

Additionally, an article is included in this issue the release of which Paul had eagerly awaited as it provides measurement uncertainty estimates reported by the participants of proficiency tests and thus it illustrates the current state of practice. Paul would call it "food for thinking".

Ernst-Heiner Korte

Editor-in-Chief