One corrosive source of indifference about such distinctions is an old dogma, perhaps falsely ascribed to David Hume: that one cannot derive an "ought" from an "is". Which can suggest the following: if anyone offers an inference to a moral conclusion from agreed facts, either the conclusion doesn't follow, or, if it does follow, the premises must state not just facts but also ethical presumptions as contentious as the conclusion. Anscombe undermined this assumption in an early essay, On Brute Facts, which was as simple as succinct. She imagined the following sequence of descriptions: "The grocer had the potatoes brought to my house and left there"; "He supplied me with potatoes"; "I owe him such-and-such a sum of money" - to which John Searle was explicitly to add, "I ought to pay him such-and-such a sum of money." Anscombe notes that none of these descriptions entails its successor: at any point, for all that is stated, something untoward might exclude the ordinary inference. And yet in normal circumstances (which couldn't be spelled out exhaustively) the inference goes through.

Anscombe's ethical positions - over abortion, contraception and war - may not be equally cogent, but all merit attention. Right-wing and left-wing politicians of a Catholic persuasion tend to be selective within their Christian inheritance. The right wing may focus on abortion, the left wing on war, poverty, and capital punishment. There may be reason to discriminate: the reach of the Church's infallibility is restricted. But Christians who rightly bring their religion into politics must be careful that their politics is serving their religion, and not their religion their politics. Here Elizabeth Anscombe, and Peter Geach, are exemplary: for them, religion comes first - which does not mean (remember Geach on the Logos) that philosophy comes second.