Well, in the case of pounds 643.75 worth of Roger Waters's princely income, I can tell you. The story goes like this. Last year, I produced a book called The Five Giants, a history of the welfare state since Beveridge, in the course of which I quoted one verse - one verse - from Pink Floyd's The Wall: the famous one about "Teachers leave them kids alone" to illustrate a point about education in the 1970s.
I also quoted lines from the Beatles' Taxman, Lionel Bart's Fings Ain't What They Used To Be, and from EJ Harburg's Buddy can you spare a dime?
A quote's a quote. And for perfectly good reasons - if you ever write a hit song you may only ever write one - songwriters, like poets, expect royalties. So my publisher, HarperCollins, and I wrote off for permission, expecting to pay a fee. Duly, the letters came back. George Harrison (yes, he wrote Taxman) wanted pounds 35. Lionel Bart pounds 30. Harburg's publishers pounds 47 for the line "They used to tell me I was building a dream..." Fair enough. Pink Floyd Music Publishers, on behalf of Mr Waters, wanted pounds 350!
Surely some mistake. After all, I'd quoted not the whole song but one verse. No one was depriving anybody of anything. Indeed, if I was doing anything, I was offering up a little free advertising, to be paid for by myself.
My editors were equally horrified. We wrote back, along the lines of "you cannot be serious". It was gently pointed out to Pink Floyd Music Publishers that this was no potential airport bodice-ripper but rather - I use my publisher's words - "a scholarly history of the welfare state since 1940". We offered them pounds 50 - a sum we pointed out would still be the largest single permission charge. No joy.
The paperback is now out, and Pink Floyd Music Publishers have been back to the trough. Another pounds 293.75 to quote the verse in the paperback edition - almost pounds 650 in all.
But perhaps I should have known. Back in 1973, Mr Waters also wrote Money for Pink Floyd's Dark Side of the Moon. Its lyrics include the immortal lines:
Money, it's a gas
Grab that cash with both hands
And make a stash
Don't give me that do goody-good bullshit
I'm in the high fidelity, first class travelling set
And I think I need a Lear jet.
At the time, I thought Mr Waters was being ironic.