Kant's correspondence includes a short letter from a well-wisher named Bertram. The content of the letter is as harmless as it is uninteresting: Bertram invites Kant to visit his brother's estate. ‘Do come,’ he says, ‘because the weather is so beautiful and such travel so beneficial’ (10: 182). The interest of the letter is entirely exhausted by the date: 20 May 1775. For Kant used the letter to scribble down ideas, some suggestive of themes later to emerge in the first Critique. Similarity of handwriting and train of thought suggests that these scribbles are part of a whole bundle of papers collectively known as the Duisburg Nachlaβ (R4674-4684 in volume 17 of the Academy Edition). It was supposedly Kant's practice to use letters as scrap paper shortly after receiving them. If this is true, we can date the Bertram scribbles - and, in all likelihood, the bundle as a whole - to some time in 1775. That would make the Duisburg Nachlaβ one of the few pieces of philosophical writing in Kant's own hand to come down to us from the 1770s, and the only extended piece in his hand from the middle of the decade. It offers a rare glimpse of Kant at work on immediate ancestors of central ideas in the first Critique and might therefore be expected to shed light on an important chapter in Kant's philosophical development.