I am going to look at the barber version of Russell's paradox but same argument will work for sets version as well. The barber version goes as:
A barber cuts hair of only those people who do not cut hair of themselves. ---(1)
Paradox arises when one asks whether barber cuts his own hair or not. A further simplified version can be written as:
For all persons that exist, a barber cuts hair of a person if and only if that person does not cut hair of self. ---(2)
Now if we designate barber with B
, a person with X
and P
cuts hair of Q
as f(P,Q)
. Then (2) basically can be written as:
∀X, f(B,X)
iff¬f(X,X)
---(3)
So if we assume f(B,B)
this will imply ¬f(B,B)
and if we assume ¬f(B,B)
this will imply f(B,B)
, which raises the paradox. Note that we need to assume either f(B,B)
or ¬f(B,B)
, otherwise that will violate the Law of Excluded Middle. Now, note that (3) is a collection of all the following statements:
f(B,X1)
iff¬f(X1,X1)
f(B,X2)
iff¬f(X2,X2)
...
f(B,B)
iff¬f(B,B)
---(4)...
f(B,Xn)
iff¬f(Xn,Xn)
The statement (4) is basically a violation of the Law of Non-Contradiction (not exactly, but one can prove it). Similarly, one can start from a logically invalid statement and reach Russell's Paradox:
f(B,B)
iff¬f(B,B)
---(5)
Now assume that all X
apart from B
satisfy:
∀X≠B, f(B,X)
iff¬f(X,X)
---(6)
Note that (6) is logically valid. So, we combine a logically invalid sentence (5) with logically valid sentence (6) to reach (3), which will then obviously be logically invalid too.
So the question is if Russell's Paradox is basically equivalent to the statement:
A
iff¬A
---(7)
so what is so great about it? In philosophical sense, what is the significance of Russell's Paradox?
If you state your original statement itself as (7) then assumption A
will lead to ¬A
and assumption ¬A
will lead to A
. Basically it does not seem like Russell's Paradox starts with something valid and logically concludes something invalid, rather it seems like it starts with something logically invalid in the first place, namely the violation of the basics of Logic.
There are numerous ways suggested to avoid Russell's Paradox by putting several kinds of restrictions; but following my argument above, we only need to disallow any statement that inherently contains (7), this way one can avoid Russell's Paradox in the set theory. Or am I misunderstanding something and the matter is more subtle than this?
Also, I will be very thankful if someone can tell me if similar discussion is done elsewhere in literature, so I can read more about it.
NOTE 1: If you think of function f(P,Q)
as P
contains Q
then you get the sets version of Russell's Paradox. In fact, f(P,Q)
can be any function for example if we choose it as "love" and B as Bess, then Russell's Paradox will become something like "Bess loves someone only if she doesn't love herself, does Bess love herself?"
NOTE 2: By slightly modifying (3), let's create our own so-called paradox as following:
∀X, f(B,X)
iff¬f(X,B)
---(8)
In Barber language, "A barber cuts hair of only those people who don't cut the hair of barber, does barber cut his own hair?" And in Sets language, "A set P only contains those sets that don't contain P, does P contain P?" So again, the question is what is so special about Russell's Paradox and this one too if they are basically equivalent to (7)?
NOTE 3: The SEP Article initially points out issues with Unrestricted Comprehension Axiom and vicious circle but in my explanation above, I see problems with Russell's Paradox in its statement and assumptions itself. Later it discusses it in Contemporary Logic but none of that seems to be similar to my argument, unless I missed something.
NOTE 4: To give an analogy to the question: Assume we have a law:
1+1=2 ---(9)
then we make the following sentence:
Two and Two Apples are Six Apples ---(10)
Now, (10) reduces to 2+2=6, divided by 2 gives 1+1=3, which directly violates my law (9). Now what is so great about (10), do we need to restrict our comprehension to avoid making statements like (10)? Similarly we have some assumed laws of logic, if Russell's Paradox contains a statement (4) that directly violates the basic laws of logic then what is so great about it? It is not much different compared to making statements like (10).
NOTE 5: Because a lot of people are not satisfied with barber version I mentioned above, here I am adding the sets version of my argument. Russell's Paradox describes a set:
R = {x | x ∉ x} ---(11)
this can be written as:
∀ x, x ∈ R ⟺ x ∉ x ---(12)
equivalently:
∀ x, x ∈ R ⟺ ¬(x ∈ x) ---(13)
this is a collection of following sentences:
x1 ∈ R ⟺ ¬(x1 ∈ x1)
x2 ∈ R ⟺ ¬(x2 ∈ x2)
...
R ∈ R ⟺ ¬(R ∈ R) ---(14)
...
xn ∈ R ⟺ ¬(xn ∈ xn)
The statement (14) is same as what I meant above in (4). Rest of the question then follows as earlier.
f(P,Q)
asP
containsQ
and you get the usual sets version of Russell's Paradox.