Most days of our lives we are required to prove who we are. But communicating our identity to someone else - without recourse to intrusive techniques like genetic fingerprinting - is a tricky philosophical problem: in what true and dependable outward manifestations does the inner self lie? Clearly, in nothing so mutable as appearance. Instead, what is needed is some external correlate of the singularity that resides within. Almost always the sign that is chosen is our signature.
On cheques, documents, instructions, receipts, orders and acknowledgements, our characteristic doodle is deemed to be a true and sufficient token of our participation and implication. For most legal, economic and historical purposes, we are nothing but our signatures. Yet how primitive it is: it harks back to an age when the ability to sign was so rare as to be almost magical. The signature was like a secret rune, an imprint of the soul. In practical terms, forgery was well-nigh impossible, since so few could write their own name, never mind anyone else's. Today, the usefulness of signatures is founded on the assumption that, although ubiquitous now, each one remains inviolably unique.
This is about to change. Technology is reaching the stage where machines will be able to produce perfect copies of any signature, correct down to the details of pen velocity, pressure and inclination to the paper. At a stroke - or several - the trustworthiness of the signature will disappear. With it will go our primary means of identifying people.
To combat this, other attributes will be investigated. Voice recognition systems are already in use; their proponents claim that they can distinguish infallibly between a human voice and an impersonation. This may be so, but related developments are subverting their usefulness. Cognate devices can emulate an input voice by reversing the procedures of recognition, synthesising sound from the resultant data. According to the original discrimination scheme, the output is necessarily identical.
A similar approach can be applied to any other human attribute. For every verification test is made up of two processes: analysis and comparison. The analysed input can therefore be used to reproduce all its salient characteristics - salient according to the test, that is - thus ensuring that the artificially constituted feature passes the test. Anything that can be measured can be mimicked; and anything that mimics as well as the test can judge is indistinguishable by that test from the original.
So it will be that for any check proposed as a way of establishing personal identity, a machine will be able to mimic the specific feature under scrutiny. Imagine, then, a machine that can simulate to the point of interchangeability every testable characteristic of a given person. The aping machine, in all such respects, is that person. Nor is this limited to the obvious tests of individuality: the same applies to intelligence, emotions and the rest of life's baggage. This is the Turing point, when the distinction between machine and human vanishes. And it will come. Enjoy the uniqueness of your signature - and your humanity - while you can.