must には「必ずこうなる」という不可避性の意味・用法がある（そして認識用法とは異なる）

ともすれば法助動詞の must には「〜しないといけない」（義務・必要）と「〜にちがいない」（認識的必然性）の2つくらいしか意味がないと言われがちですが，これらからはっきり区別されるべき「不可避性」(inevitability) の意味もあります．

たとえば，こんな例：

a. What this means is that a lot of "demand destruction" must take place over the next few months. (Paul Krugman, "A Pig In A Jacket," New York Times, Oct 7, 2005)
b. Even optimists generally concede that the housing boom must eventually end, and that consumers will eventually have to start saving again. (Paul Krugman, "Intimations of Recession," New York Times, August 7, 2006)
c. The General Theory [...] led to the conclusion that light must be attracted by heavy bodies. 〔一般相対性理論は…光は重い物体に牽引されるにちがいないという結論に到った〕 Einstein saw that if this were correct then light which travels close to the sun on its way from a star to the earth must be deflected by the gravitational pull of the sun. 〔アインシュタインは，もしこれが正しいとしたら 太陽の近くを通って星から地球に向かう光は太陽の重力に引き寄せられて必ず屈折されると考えた．〕
(Bryan Maggee, Popper (3rd ed), Fontana Press, 1973=1985, pp.43-4)

いずれも，人為的になにかをなす必要・義務ではありえない点に注意してください．

誤解されやすいところですが，この不可避性の意味は，「ちがいない」という推量とは異なます．両者をわかつ特徴としては，must の表す不可避性は未来指示になりうるのに対し，must の推量用法は基本的に未来に言及できない点が挙げられます: e.g. The plane {should/*must} land in a few minutes. (Rivière 1981: 181) ）

古い文献ですが，この不可避性の意味・用法は Tregidio (1982) で紙幅を割いてとりあげられています：

Tregidgo, P. S. (1982) "MUST and MAY: Demand and permission," Lingua 56, 75-92.

追加

a. All men must die. ('Nature or God demands that this should happen.')
b. You must go poking your nose into everything! ('Your character demands that this should happen.').
c. Men must work and women must weep. ('Life or the world demands it.')
d. For the English historian it must have a peculiar importance. ('The nature of the fact and of English historians demands that it should be so.')
e. If there is a good spark and the right amount of petrol, the engine must start. ('The circumstances demand it.')
f. Smith is unmarked on the left - he must score! ('The situation demands it.')
(Tregidgo 1982: 79)

Tregidgio のコメント：

Here, MUST indicates an inevitable consequence, and is similar in meaning to can't help, e.g. Men cannot help but die, you can't help poking your nose into everything, and so on.
(Tregidgo 1982: 80)