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COMPUTING SCIENCE

Identity Crisis

Brian Hayes

Suppose I had once borrowed your boat and, secretly, replaced each board with a similar but different one. Then, later, when I brought it back, did I return your boat to you? What kind of question is that? It's really not about boats at all, but about what people mean by "same."

—Marvin Minsky, The Society of Mind

I have heard of a touchy owner of a yacht to whom a guest, on first seeing it, remarked, 'I thought your yacht was larger than it is'; and the owner replied, 'No, my yacht is not larger than it is'.

—Bertrand Russell, "On Denoting"

The equal sign seems to be a perfectly innocent bit of mathematical notation. We all know exactly what it means. The symbol "=" is the fulcrum of a balance. It declares that the things on either side of it, whatever they may be, are equivalent, identical, alike, indistinguishable, the same. What could be clearer? Although an equation may be full of mystery—I can't explain what e = –1 really means—the enigma lies in the two objects being weighed in the balance; the equal sign between them appears to be totally straightforward.

But equality isn't as easy as it looks. Sometimes it's not at all obvious whether two things are equal—or even whether they are two things. In everyday life the subtle ambiguities of identity and equality are seldom noticed because we make unconscious allowances and adjustments for them. In mathematics they cause a little more trouble, but the place where equality gets really queer is in the discrete, deterministic and literal-minded little world of the digital computer. There, the simple act of saying that two things are "the same" can lead into surprisingly treacherous territory.

What follows is a miscellaneous collection of problems and observations connected in one way or another with the concepts of equality and identity. Some of them are mere quibbles over the meaning of words and symbols, but a few reflect deeper questions. The difficulty of defining equality inside the computer may even shed a bit of light on the nature of identity in the physical world we think we live in.

Some Are More Equal Than Others

There's an old story about the mathematician who sets out to learn a computer programming language such as FORTRAN or C. Everything goes swimmingly until she comes to the statement x = x + 1, whereupon she concludes that computer programming is mathematical nonsense.

Of course this story is just a programmer's joke at the expense of mathematicians. I would respond in the same spirit by suggesting that a mathematician would be better equipped than anyone else to solve the "equation" x = x + 1. Obviously x is equal to X0, the infinite ordinal number, which has just the property that X0 = X0 + 1.

In truth, x = x + 1 is not an equation at all in FORTRAN or C, because the symbol "=" is not an equal sign in those languages. It is not a relational operator, comparing two quantities, but an assignment operator, which manufactures equality. When an assignment statement is executed, whatever is on the left of the "=" sign is altered to make it equal to the value of the expression on the right. The semantics of this operation are altogether different from those of testing two things for equality. As it happens, the semantics of assignment introduce certain troublesome characteristics into computer programs, which I shall have occasion to mention again below.

To avoid confusion between equality and assignment, many programming languages choose different symbols for the two operations. For example, Algol and its descendants write ":=" for assignment. In all that follows the symbol "=" will mean only equality (whatever that means).

By the way, the "=" notation was invented by Robert Recorde (1510–1558). He chose two parallel lines as a symbol of mathematical equality "because noe 2 thynges can be moare equalle."



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