Full Idea
That which is necessary, that which must be, means that which will be, whatever suppositions we may make in regard to all other things.
Gist of Idea
Necessity is what will be, despite any alternative suppositions whatever
Source
John Stuart Mill (System of Logic [1843], 3.06.6)
Book Reference
Mill,John Stuart: 'System of Logic (9th ed, 2 vols)' [Longmans, Green etc 1875], p.392
A Reaction
[Mill discusses causal necessity] This is quoted by McFetridge. This slightly firms up the definition as 'what has to be true', though it makes it dependent on our 'suppositions'. Presumably nothing beyond our powers of supposition could matter either.