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Named after Bertrand Russell, an English Mathematician and Philosopher.

The paradox deals with how Sets can be defined.

The formal statement of the paradox is given at the foot of this page but here is a more informal example.


Consider the word "pentasyllabic" (means a word with five syllables). It's interesting because it
describes itself, that is, it has five syllables. We shall call words that describe themselves HOMODOXIC.

Now consider the word "peanut." It doesn't describe itself. We call words that don't describe themselves
HETERODOXIC. All words are either HOMODOXIC or HETERODOXIC, and never both.

We now make two sets:
* All HOMODOXIC words = {pentasyllabic, ...}
* All HETERODOXIC words = {peanut, ... }

In which set should be we place the word HETERODOXIC? It's got to be in one or the other.

If it is in the first set, then the word HETERODOXIC has the property of being HOMODOXIC,
and therefore it doesn't describe itself. It should be placed in the second set.

But if it is placed in the second set, the word HETERODOXIC has the property of being HETERODOXIC
and therefore it does describe itself. It should be placed in the first set.

OUCH !!!


Another even more informal example.

Consider a small village where there are two sorts of men - those who always shave themself and those who are always shaved by the barber.

In which set is the barber?

(No, he doesn't have a beard as there are no non-shavers in the village.)


Enrichment Task

Can you think of any other Homodoxic words? Clue: Colour and Color.


In formal Mathematical notation, Russell's paradox is:

Suppose that for any coherent proposition P(x), we can construct a set EQN:\{x:P(x)\}.

Let EQN:S=\{x:x\not\in~x\}.

Suppose EQN:S\in~S ; then, by definition, EQN:S\not\in~S.

Likewise, if EQN:S\not\in{S}, then by definition EQN:S\in{S}. Therefore, we have a contradiction.