Rembrandt has drawn several portraits of Jan Six next to an open window, often engrossed in some activity, as his writing above. It seems that Jan Six has that rare quality of being an outdoor’s man, with introspective sensitivities.
In this drawing, there is the sense of interiors and exteriors which he captures both with the actual subject of the drawing (Jan Six Writing), and with the formal composition.
The beautiful landscape which we glimpse through the open window suggests that Jan Six is engaged in some kind of mesmerizing world of his imagination while writing this letter. Who is it to, we ask; what is it about, is Jan Six longing for something, an encounter with someone, who resembles the beauty of this exterior landscape?
Now, to the composition of the drawing.
There are many "squares" in the picture: the square of the table, the square of the window, the square of the window shade, and the square of the letter.
Jan Six, as the protagonist of the drawing, shows us his importance by round hat he's wearing (who wears a hat indoors, to write a letter?). But, we know from this round hat, different from all the squares, that he's the focus of the attention.
The squared, flat table, on which sits the squared letter, mimics the squared vertical window opening.
From this we make the association that the inner world on the table and the letter, is linked with the outer world of the countryside by these converging squares.
Rembrandt draws an abstract picture (of squares and circles) to unite an emotional and visual one to build our imagination around a small, intimate story about a young man writing a letter.