# singleton sets

```Here's someone else who doesn't like singleton sets,
and hence doesn't like classes which are individuals.

John Barwise & John Etchemendy (1992), "The Language of First-Order Logic",
Third Edition, Revised & Expanded, Center for the Study of Language and
Information, Stanford, Page 212

Suppose there is one and only one object x satisfying P(x).  According
to the
Axiom of Comprehension, there is a set, call it a, whose only member is x.
That is,
a = {x}.  Some students are tempted to think that a = x..  But in that
direction lies,
if not madness, at least dreadful confusion.  After all, a is a set (an
abstract object)
and x might have been any object at all, say Stanford's Hoover Tower.
Hoover is
a physical object, not a set.  So we must not confuse an object x with the
set {x},
called the singleton set containing x.  Even if x is a set, we must not
confuse it with
its own singleton.  For example, x might have any number of elements in it,
but {x}
has exactly one element: x.

Dick McCullough
Ayn Rand do speak od mKR done;
mKE do enhance od Real Intelligence done;
knowledge := man do identify od existent done;
knowledge haspart proposition list;
http://mKRmKE.org/
```

Received on Tuesday, 12 August 2008 21:09:16 UTC