In an interview with Gabriella Poli in 1976, Primo Levi explained that all his books had been born in pairs, as twins: two books on his Lager experience - If This is a Man (Se questo è un uomo, 1947) and The Truce (La tregua, 1963) - were followed by two collections of science-fiction short-stories - Storie naturali ('Natural Histories', 1966) and Vizio di forma ('Formal Defect', 1971) (partly collected in a single volume in English, The Sixth Day). The unconventional and heterogeneous (auto)biography of an inorganic chemist, The Periodic Table (Il sistema periodico, 1975) was planned to be paired with a book on the trade of an organic chemist, with the working title of Il doppio legame ('The Double Bond'). Levi never completed this book; instead he paired The Periodic Table with the fourteen stories of The Wrench (La chiave a stella, 1978), centred on a highly skilled Piedmontese master rigger, Libertino Faussone. This particular coupling might look looser than the others, but it has striking significance for our understanding of Levi's work. Its prime effect is to underscore the challenge Levi presents to abstract, systematic notions of scientific knowledge (implicit in the image and concept of the 'sistema periodico'), in favour of the technical, applied skills common to both the (inorganic) chemist and the rigger. At the centre of Levi's concerns in these two books, in other words, is the idea of knowledge as being both a material and existential construction rather than a purely abstract or theoretical one, as well as a notion of the unity of art (as craft) and technology, at both the experimental and manual levels. Taken together, the diptych of The Periodic Table and The Wrench offers one of the most interesting of modern representations - in philosophical, epistemological, aesthetic and ethical terms - of what Levi would call the homo faber, man as maker or tool-maker, the fabricator and artist who works upon hard material such as stone or wood.