Lord Blackburn said,

‘My Lords, it is of great importance that those principles should be ascertained; and I shall therefore state, as precisely as I can, what I understand from the decided cases to be the principles on which the Courts of Law act in construing instruments in writing; and a statute is an instrument in writing.

In all cases the object is to see what is the intention expressed by the words used. But, from the imperfection of language, it is impossible to know what that intention is without inquiring farther, and seeing what the circumstances were with reference to which the words were used, and what was the object, appearing from those circumstances, which the person using them had in view; for the meaning of words varies according to the circumstances with respect to which they were used.

I do not know that I can make my meaning plainer than by referring to the old rules of pleading as to innuendoes in cases of defamation. Those rules, though highly technical, were very logical. No innuendo could enlarge the sense of the words beyond that which they primâ facie bore, unless it was supported by an inducement or preliminary averment of facts, and an averment that the libel was published, or the words spoken, of and concerning those facts, and of and concerning the Plaintiff as connected with those facts. If those preliminary averments were proved, words which primâ facie bore a very innocent meaning might be shewn to convey a very injurious one…

In construing written instruments I think the same principle applies…

In the case of a contract, the two parties are speaking of certain things only, and therefore the admissible evidence is limited to those circumstances of and concerning which they used those words… In neither case does the Court make a will or a contract such as it thinks the testator or the parties wished to make, but declares what the intention, indicated by the words used under such circumstances, really is.

And this, as applied to the construction of statutes, is no new doctrine…’

[Edited for ease of reading. You can find the original case on Westlaw].