London's Conservative mayor has made this well-publicised speech about elitism, equality and greed (http://m.bbc.co.uk/news/uk-politics-25135225), which I have not read in full and on which I therefore cannot really comment, in which he referred to: "human beings who are already very far from equal in raw ability, if not spiritual worth".
Now, if I said to you: "I want your essay handed in by Sunday, if not sooner," it would be clear to you what I meant. If I said: "It is a good restaurant, if not as good as it might be," then you'd know clearly what I meant.
But these two "if not"s have contradictory meanings; one means "it is" and the other means "it isn't". So it is a dangerous form of words for a controversial speech about a complicated political issue - a speech in which the meaning of words matters.
I know that when Boris Johnson says "human beings who are already very far from equal in raw ability, if not spiritual worth", he is referring to: "human beings who are already very far from equal in raw ability, despite their obviously being of equal spiritual worth."
But the words on the page sound as if he could have meant: "human beings who are already very far from equal in raw ability, and indeed far from equal in spiritual worth," which sounds like a passage from Brave New World as re-written by Sir Keith Joseph and is not what Mr Johnson meant at all.
I am surprised, if not very surprised, that his speechwriters let this line slip through.