By the time our course starts, in 1600, Europeans were actually churning out a lot of documents about their private lives. Just as we do today, they had to use lawyers and contracts for a lot of the ordinary business of life: dividing up an inheritance, buying, selling, and renting property, giving money to their kids. They also used writing for more personal purposes, for letters, diaries, and memoirs. A book like The Economy of Europe is based on these kinds of documents, whether directly (on topics that de Vries himself has researched) or indirectly (based on the research of others, which he has summarized).
There are millions of pages of these materials, and historians will probably never read all of them; certainly we're nowhere close at this point. That's one reason why there are debates among historians: new materials keep being discovered, and these force us to rethink problems we thought were settled.
Here are some photos of documents from early seventeenth-century France, to give you a sense of what a historian like de Vries is working with. One is an agreement between a wealthy mother and her adult daughter about an inheritance; it lists the goods in the family's Paris mansion. Notice how elegantly both women sign the document (the other signature is by the family's lawyer).
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This is the deathbed testament of another woman from the same family, from 1646:
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The next is a page from a rental agreement between a landowner and a rich tenant farmer, in southwest France.
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These documents were written on paper, standard procedure in the early seventeenth century; because paper was then made entirely out of rags, it was very high quality and has lasted beautifully. But as late as 1550, documents like these were written on parchment, made from sheepskin and much more expensive than paper, like the contract below (it deals with the rental of a big estate):
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Notice how much more careful the handwriting is; only a professional could produce handwriting like this, and it would have taken him a lot of time to do so. As written documents became more commonplace, handwriting became faster and less formal, and more people produced it. Because paper was much cheaper than parchment, written documents were less of a major production and much more widely used.
Documents like these were drawn up by notaries, legal specialists akin to our business lawyers; they knew what formalities and terminology had to be used if an agreement was to be valid. Here are a couple of pictures of notaries at work in their offices, one from Germany in the later sixteenth century, the other from Holland in the later seventeenth. There would have been offices like these in every European city, and a big city like Paris had dozens; even many villages had notaries. As the images make clear, business was booming!
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