The Hague, with the court, was an early example, where artists split into two groups in 1656 with the founding of the Confrerie Pictura. With the obvious exception of portraits, many more Dutch paintings were done "speculatively" without a specific commission than was then the case in other countries – one of many ways in which the Dutch art market showed the future. There were many dynasties of artists, and many married the daughters of their masters or other artists. Many artists came from well-off families, who paid fees for their apprenticeships, and they often married into property. Rembrandt and Jan Steen were both enrolled at the University of Leiden for a while. Several cities had distinct styles and specialities by subject, but Amsterdam was the largest artistic centre, because of its great wealth. Cities such as Haarlem and Utrecht were more important in the first half of the century, with Leiden and other cities emerging after 1648, and above all Amsterdam, which increasingly drew to it artists from the rest of the Netherlands, as well as Flanders and Germany. Dutch artists were strikingly less concerned about artistic theory than those of many nations, and less given to discussing their art; it appears that there was also much less interest in artistic theory in general intellectual circles and among the wider public than was by then common in Italy. As nearly all commissions and sales were private, and between bourgeois individuals whose accounts have not been preserved, these are also less well documented than elsewhere. But Dutch art was a source of national pride, and the major biographers are crucial sources of information. These are Karel van Mander (Het Schilderboeck, 1604), who essentially covers the previous century, and Arnold Houbraken (De groote schouburgh der Nederlantsche konstschilders en schilderessen – "The Great Theatre of Dutch Painters", 1718–21). Both followed, and indeed exceeded, Vasari in including a great number of short lives of artists – over 500 in Houbraken's case – and both are considered generally accurate on factual matters. The German artist Joachim von Sandrart (1606–1688) had worked for periods in Holland, and his Deutsche Akademie in the same format covers many Dutch artists he knew. Houbraken's master, and Rembrandt's pupil, was Samuel van Hoogstraten (1627–1678), whose Zichtbare wereld and Inleyding tot de Hooge Schoole der Schilderkonst (1678) contain more critical than biographical information, and are among the most important treatises