The infamous note carried by Corporal Nobbs to prove that he is a human being.
The note reads:
I, after hearing evidence from a number of experts, including Mrs Slipdry the midwife,
certify that the balance of probability is that the bearer of this document, C. W. St John Nobbs
is a human being.
- Signed, Lord Vetinari.
Of course, no one believes that it is real...