Nowadays, it is quite common to remark that theory and practice are two different things. For many artists – and art historians – theory is nothing more than mere speculation. Marcel Duchamp (1887-1968) considered his paintings and ‘ready-mades’ the practical productions of a theory that he was thinking about and working with. In 1972, Joseph Beuys (1921-1986) outlined this modern conception of theory by exhibiting his Plastische Theorie on a blackboard, as if his ‘theory’ was in fact a kind of schoolroom course [Fig. 4]. In this context, theory describes first of all an intellectual and abstract quality, that serves the purpose, according to Arthur C. Danto, of legitimising soap boxes as a work of art, like the Brillo Boxes (1964) by Andy Warhol (1928-1987).

In our modern and post-modern world, it seems to be possible to understand the theories of works of art without seeing them, as if the theory was the prerequisite of the practice. But was this always the case? And was it especially true for the seventeenth century? What did it mean for a painter like Samuel van Hoogstraten to write a theory about art – and, especially, about his art? He is one of the first in the Dutch artistic literature to use the word ‘theory’: