The invention of double-entry bookkeeping has had profound implications on our world. All modern commerce owes a debt (hi!) to it. I have heard it compared to the invention of numbers. In turn, this accounting mechanism was made possible by the sudden availability of cheap paper.

In The notebook: a history of thinking on paper, Roland Allen writes:

Before paper became easily available in the years after 1244, Italian merchants had used parchment books to record their transactions, but they recognised a superior product when the first paper ledgers arrived. Ink dries on the surface of parchment, but soaks into a paper sheet. This makes it a permanent medium, whereas parchment, which can be scraped clean and written on again, allows records to be revised after the event—opening the door to fraud. Accounts were always bound into ledgers for a similar reason: loose-leaf entries could easily be fabricated, but a ledger with numbered pages became tamper-proof. This in turn meant that merchants could delegate to subordinates or branch offices without fear of embezzlement, allowing traders to widen the circle of their businesses. Purchases, sales, loans and terms of business were no longer recorded inconsistently on a scrap of easily rewritten parchment: they were carefully entered into a permanent record that, in the event of dispute, was accepted as evidence in a court of law.

The new techniques of double entry and cross-reference increased the number of entries that a merchant had to make, meaning that ever more ledgers became necessary. This was only affordable if paper was cheap: and now, for the first time, it was.

And then:

So paper became cheap enough for a minor merchant to have six or seven ledgers with hundreds of pages each.

In July, OpenAI slashed the prices of their GPT Luna model:

Starting today, we are reducing prices for GPT-5.6 Luna by 80%

The new prices are $0.10 per million input tokens and $0.50 for output. In comparison, their flagship model, Astra, costs $10/$50 for the same. When I first used Luna with this new pricing, I thought the cost counter was broken. I had been programming all day and it was still at zero. Luna is fast and suddenly cheap—just like writing material centuries ago.

What new industries will this create? What massive, society-changing concepts—like double-entry bookkeeping—can we invent?

What will you build?