LONDON — The chancellor didn’t use the word in his budget statement and only made passing reference to the forthcoming negotiations, but Brexit hovered over everything Philip Hammond said in his first budget statement.

Yet those looking for clues as to how the British government is planning for their impending divorce from the European Union will be disappointed.

Why? Well no one knows what Brexit will look like. We don’t know what its impact on the U.K. economy will be. In this context, every figure Hammond set out had an air of unreality to it.

What, for instance, will be the size of the Brexit divorce bill the EU will try to extract? The full £52 billion as some on the Continent have suggested? That would be almost half the annual budget of the NHS. Not an insignificant sum. As the Institute for Government has pointed out, even half that sum would take a big chunk out of the government’s plan to cut borrowing, forecast to fall to £40.8 billion in 2018-2019, according to the Office for Budget Responsibility, we learned today.

The chancellor can’t really be blamed for any of this. He is dealing with “known unknowns.” Treasury officials said they are still not even certain that the Brexit divorce bill “exists” — so how to plan for it? In their forecast, just published, the OBR writes “no allowance … can be made [for it] until more information becomes available.”

And what about the macroeconomic impact of Brexit? At last year’s Autumn Statement, the OBR had very little to base its projection on. (This was before May’s Lancaster House speech.)

The OBR’s new forecast, despite being able to take into account the government’s negotiation strategy as set out by May, makes no major new assumptions because “there is no meaningful basis for predicting the precise end-point of the negotiations as a basis for our forecast,” the document states. Even though we now know the U.K. will leave the single market, we have no idea what the shape of a free-trade agreement will be.

We have no idea whether the Brexit talks will be successful, or concluded in time to avoid the U.K. crashing out of the EU on WTO rules with no deal.

We have no idea if, or for how long, the country will continue to trade with the EU under a transitional arrangement.

We have no idea how successful the U.K. will be in securing beneficial trade deals with other countries.

In short, there’s a lot that Hammond, and everyone else, does not know. Without taking the shine off his big day, today’s Red Book is, more perhaps than ever before, a work of speculation.

This insight is from POLITICO’s Brexit Files newsletter, a daily afternoon digest of the best coverage and analysis of Britain’s decision to leave the EU. Read today’s edition or subscribe here.