The difficulties of predicting future threats
So the government has revealed what it thinks are the most serious threats to UK security and what it might do about them.
One colleague, leaving the Whitehall briefing where officials explained their layered approach, joked it was "tiers for fears".
Tier One, the gravest, included terrorism, cyber attack, a major natural disaster, or a foreign war drawing in this country.
Officials explained that "judgements had been made", in weighting the relative impact and likelihood of a whole variety of possible challenges, and this explained why others had been relegated to Tier Two or Three.
In nailing its assessments to the mast in this way, the National Security Council - itself a product of the new government, wanted to avoid accusations of producing the kind of "all shall have prizes" Whitehall paper in which every single priority within every departments is accorded a few words underlining how terribly important it is.
The downside to this approach was that the ranking of these individual threats and why, for example, the UK being nuked by a foreign power was classified as Tier Two, would become the subject of intense argument.
How far Cabinet Office judgements have been swayed by the desire to save money is open to question.
Clearly the authors would bridle at the idea that they designed their conclusions to allow budget cuts, but given the Downing Street foreword notes, "our ability to meet current and future threats depends crucially on tackling the budget deficit", they did not presumably have the option of saying that the increasingly broad range of threats they have identified require more to be spent on defence and intelligence.
The resulting judgements will be argued with by many. Terrorism or cyber attack might be very serious issues but would they really threaten UK national survival or the functioning of this society?
Even a 9/11 scale event or the shutting down of the City by cyber attack, enormous as such traumas might be, would be trivial in importance compared to a nuclear attack by a foreign state.
Obviously a nuclear attack by a foreign state is far less likely, and the government will argue that by replacing Trident they are already taking preventive measures, but such an event would threaten the survival of this country.
It is almost an axiom of defence planning that the more devastating a possible event, the less likely it is to happen, and the more expensive it is to prepare for.
It is in the area of a general insurance against unforeseen contingencies that the armed forces have functioned since the Cold War. In setting aside this idea, the National Security Council is taking deliberate, calculated, risks.
The idea of a state like Iran acquiring missiles in the future with ranges capable of hitting the UK with chemical or even nuclear warheads is quite plausible.
But the UK has no ballistic missile defences (they are horrendously expensive...) and little capability to respond short of nuclear retaliation. Deterrence, like some of the judgements in today's paper, assumes rationality on the part of opponents.
It is the very difficulty of predicting future threats that has caused history to be littered with examples of cutbacks that seemed like a good idea at the time - Britain's decision to run down tank and fighter production to almost nothing in the 1920s or a previous Conservative government's pledge to scrap the Royal Navy's carriers in 1981, less than one year before Argentina invaded the Falklands are just two that spring to mind.
For everyone's sake, we must just hope that the embrace of risk in today's paper and tomorrow's Strategic Defence and Security Review proves a little more fortunate.