Essays on Philosophy

Russell's Paradox in Frege's System

2010-01-31T17:36:00.001+08:00

Russel had told Frege his paradox. In Frege's system, the paradox has a different formulation. Frege defined a concept as a function that has a truth-value. Frege used the notion of the extension of a concept. Frege did not define extensions, but used axioms to govern them. The extension of a concept can be understood as the mapping in mathematics, which is considered as the ordered pairs of arguments and value of the function. That is to say, it tells us the arguements for the function having the True as the value, and also the arguments for it having the False as the value. Let us designate the extension of a concept F as εF. One of the famous axioms in Frege's system is the Basic Law V:

εF = εG if and only if ∀x[F(x) = G(x)].

The equality in the left-hand side means the concepts F and G have the same extension (as an object), while the equality in the right-hand side means F(x) and G(x) have the same truth values for all arguments x. Let us consider a concept H(x) defined by

∃G[x = εG ∧ ¬G(x)]

(there exists G such that x = εG and not G(x)). Then H(εH) (is true) if and only if ∃G[εH = εG ∧ ¬G(εH)]. If ∃G[εH = εG ∧ ¬G(εH)], it follows from Basic Law V that ¬H(εH). On the other hand, it is clear that if ¬H(εH), then ∃G[εH = εG ∧ ¬G(εH)] (where G is taken to be H) so that H(εH). The latter implication (without Basic Law V) shows that the definition of H may be doubtful, so that the possibilities will be:

¬H(εH) is false, while (it follows from the former implication that) it is the problem of the Basic Law V;
H(εH) is meaningless (either εH is not in the range of significance of H, or H(x) is meaningless for any x).

First of all, we may say two concepts to be extensionally equal if they have the same range of significance, and have the same values on the range. At least a concept is extensionally equal to itself. Then the next step will be the formulation of (the existence of) such an object called extension (before we can tell whether it is in the range of significance of some concept) such that the extensions of two concepts are the same if and only if the concepts are extensionally equal.

Although the existence of the extension of a concept (as an object) is taken for granted in an axiom in Frege's system, we don't know much about this object, e.g., how we can check if it is in the range of significance of another concept. Let us go back to the concept H above. How can we check if H(εH) is significant? First and foremost, we should have made sure what we mean by the range of significance of a concept. As a concept is a function that has a truth-value, the range of significance of a concept should be the set of all objects such that taking these objects as argumentsthe concept has values (either True or False). Can we say H(εH) is true or false? Can we check it against the definition of H? Can we check the case that G is taken to be H? We cannot, not only because we don't know what ¬G(εH) is when G is taken to be H, but also because we don't know if G can be taken to be H - does such an H exist anyway?

Russell's Paradox and Cantor's Theorem

2010-01-31T16:31:00.003+08:00

Russell's paradox asks whether the "set" R defined by

R = {x: x is not an element of x}

is an element of itself. As it was mentioned in a previous article, Russell's solution to his paradox was his theory of types. He arranged all propositions into a hierarchy. The lowest level of the hierarchy consisted of propositions about individuals, not sets. The next lowest level consisted of propositions about sets of individuals. The next lowest level consisted of propositions about sets of sets of individuals, and so on. Thereby the definition (one of these "valid" propositions) of any set only referred to objects of the same "type" (at the same level).

It seems that Russell discovered the paradox as a result of his analysis on Cantor's theorem on the cardinality (i.e. the "number of elements") of the power set (i.e. the set of all subsets) of a set. In the following discussion, let P(A) denote the power set of a set A. Furthermore, some common axioms of the set theory are assumed.

Cantor considered if there was uncountable set. A infinte set is countable if it can be put into one-to-one correspondence with the set ℕ of all natural numbers. For example the set of all even numbers is countable (considering the mapping that maps a natural number n to 2n), so that the set of all even number has the same cardinality (the same "number of elements") as ℕ. A infinite set is said to be uncountable if it is not countable. Cantor considered the set of all infinite sequences of elements that were either 0 or 1. For instance (0, 0, 0, ...) is one of these sequences. Clearly, it is infinite. Cantor used his diagonal argument to prove that this set is uncountable. That is, any mapping from ℕ to this set cannot be a one-to-one correspondence. Let this set be denoted by S. Consider any mapping f: ℕ → S. Then for each natural number n, we have a sequence f(n), and we denote it by (f(n)₀, f(n)₁, f(n)₂, ...). Along the diagonal, we then have a sequence (f(0)₀, f(1)₁, f(2)₂, ...). Cantor obtained a sequence (a₀, a₁, a₂, ...) in S such that a_n ≠ f(n)_n for each natural number n (if f(n)_n is 0, then a_n is taken to be 1). Thereby this sequence is different from f(n) for every natural number n. Hence, S is uncountable.

With this argument, Cantor had actually proved that P(ℕ), the power set of ℕ, was uncountable. In fact the set S is in one-to-one correspondence with P(ℕ) - for a sequence (a₀, a₁, a₂, ...) in S, consider the subset A of ℕ such that A contains n if (and only if) a_n = 1. Thus we can "translate" the diagonal argument as follows. Consider any f: ℕ → P(ℕ). Then for each natural number n, we have a set f(n) of natural numbers, so that we can ask if f(n) contains a given natural number m. Thus we can define a set A such that A contains a natural number n if and only if f(n) does not contains n. That is,

A = {n ∈ ℕ: n ∉ f(n)}.

Then for any natural number n, A ≠ f(n). That is, f is not surjective. Hence, P(ℕ) is uncountable.

It is clear that in the above argument ℕ can be replaced by any non-empty set, and we can conclude that there is no surjection from a set to its power set.

It follows immediately that there does not exist a set of all sets. Otherwise, if U was a set of all sets, then P(U) would be a subset of U, and we would have a trivial surjection f: U → P(U) such that f(x) = x for all x belonging to P(U), and f(y) was any subset of U, such as the empty set, for all y not belonging to P(U). It then leads to a contradiction that for all x ∈ U, f(x) ≠ A, where

A = {x ∈ U: x ∉ x}.

It is in fact Russell's paradox.

Tractatus: Being Thinkable

2009-07-13T01:42:00.002+08:00

As it was mentioned in a previous article on causality, some laws such as the principle of sufficient reason are necessary for people. Wittgenstein says in Tractatus (6.361) that only connexions that are subject to law are thinkable. Without laws, we could not even describe the world (in the sense of 6.341), even though art could make things manifest.

Can we compare a process with 'the passage of time'? Wittgenstein says (6.3611) that we even cannot say there is such thing called 'the passage of time' (no matter whether there is such thing, or whether it makes itself manifest). We can describe the lapse of time only by relying on some other process. Time as the temporal dimension with orientation (from the past to the future) is just something in physics to describe the world. As Einstein says, time is relative. By relying on some other processes such as the rotation of the Earth and its revolution around the Sun, we call it 'the passage of time'.

Something exactly analogous applies to space (6.3611). All events occur in the so-called space-time continuum. People say that a metal bar will expand or contract in response to a change in its state such as temperature and pressure. Without such changes, neither of expansion and contraction (which exclude one another) can occur. People think that it is because there is nothing to cause the one to occur rather than the other, while Wittgenstein says (6.3611) that it is really a matter of our being unable to describe one of the two events unless there is some sort of asymmetry to be found. An asymmetry such as an increase in temperature is then regarded as the cause of the occurrence of expansion and the non-occurrence of contraction. It is we who call such an asymmetry a 'cause' to describe the world.

Another example is the motion of an object. The movements of an object in different directions at the same time are mutually exclusive events. The orientation of the three-dimensional space is something in physics. As Einstein says, position is relative. Motion can only be observed and measured relative to a frame of reference, such as the Earth, the Sun, or the Milky Way Galaxy. 'Orientation' is just something in our thought to describe the world. Gravity is one of the asymmetries that we regard as the causes of motion. Nevertheless, Newtonian mechanics describes it as a force, while general relativity ascribes it to the curvature of space-time. All the same, they are just different theories - different descriptions of the world.

As a comment on 6.3611, 6.36111 gives a purely spatial example: a right-hand glove could be put on a left hand if it could be turned round in four-dimensional space. In our world, it is the orientation that makes the completely congruent right hand and left hand not to coincide. However, we can describe the direction of an object only by relying on the position of some other object.

Furthermore, we also see that the ways in which connexions are thinkable depend on the laws they are subject to. Rotation in four-dimensional space is thinkable only in mathematics. How can we imagine such a four-dimensional space, not to mention a rotation in such a space? What Wittgenstein mentions are just analogies in the lower dimensional spaces. As an analogy, we can imagine the rotation of the right hand around a cross-section. During the rotation, only this cross-section as the rotation 'axis' remained still in our three-dimensional space, so that we could see only the cross-sections of the veins, the flesh, and the bones, while all other parts of the hand disappeared (from our three-dimensional space). If there was light in the four-dimensional space that projected the three-dimensional shadow of the hand to our three-dimensional space, then we could see the hand (its shadow) shrinking toward the cross-section of rotation 'axis', becoming flat, then inflating from the cross-section in the opposite direction, and finally becoming identical with the left hand. That is all we can think of.

By the way, String theory claims that our universes has more than four dimensions (space-time). However, these other dimensions are so small (highly curved), not only that the right-hand glove cannot be turned round, but also that we are not even aware of them. Still it is merely a theory - a description of the world.

I cannot think beyond my thought, but I can only remind myself of my own limitation.

Tractatus on Causality

2009-07-02T01:17:00.004+08:00

From Tractatus' logical point of view, outside logic everything is accidental (6.3). It is not at all surprising. To say so, we must first clarify what it means by accident. If we define an accident / contingency to be a fact (what is a fact anyway?) that are not necessarily true or necessarily false (a priori), then such a statement becomes clear. (Of course we shall thereby ask what it means by something being necessary / a priori. It may turn out that the claim at 6.3 just follows immediately from a definition!) As it is stated in Tractatus,

in logic nothing is accidental (2.012);
there are no pictures that are true a priori (2.225);
all deductions are made a priori (5.133);
this is connected with the fact that no part of our experience is at the same time a priori (5.634).

According to Tractatus, propositions in logic have no sense, not to mention the truth or falsity of the senses (e.g. 6.111). On the other hand, the totality of true propositions is the whole of natural science (4.11).

Furthermore, there is no causality in logic (5.135 and 5.136). We cannot infer the events of the future from those of the present. Belief in the causal nexus is superstition (5.1361). However, in natural science there are laws of the causal form, 'law of causality' (6.321). Buddhists believe in Karma, while Schopenhauer considered the principle of sufficient reason a priori. As stated at 6.34, the principle of sufficient reason is a priori insight about the forms in which the propositions of science can be cast.

Aristotle also says in Posterior Analytics that to know a thing’s nature is to know the reason why it is (Book 2, Part 2), and we think we have scientific knowledge when we know the cause (Book 2, Part 11). Natural science cannot tell us anything that happens for no reason. Consider the law of thermal expansion that states how the length of a metal bar changes according to the changes of its temperature. To an effect, there may be more than one cause. The length of a metal bar will also be changed if it is being hammered on the ends. When a physicist states such a law, he / she is in fact assuming implicitly not only that other causes to the effect do not happen, but also that the effect must be happen for some reason. Thermodynamics can tell you the length of the metal bar after the change of its temperature if you know its initial length, assuming all other conditions such as pressure remain unchanged. If a metal bar would expand without any reason, how could thermodynamics predict the length of such a metal bar? Sciencists, including quantum physicists, also believe in the principle of sufficient reason (even though God does throw dice).

Wittgenstein then says at 6.41 that:
For all that happens and is the case is accidental. What makes it non-accidental cannot lie within the world, since if it did it would itself be accidental. It must lie outside the world.

Besides, Wittgenstein in fact has neither told us what in the world are states of affairs, nor given us any examples. It is because the facts, and the existence of states of affairs, are all accidental. Even the existence of the world is accidental. Not only Existentialists like Sartre think so! (Of course Existentialists mean differently by something being accidental.)

In Tractatus (philosophy), Wittgenstein tries to draw a limit to not only what can be said (natural science), but also what is not accidental (logic).

The General Form in Tractatus

2009-05-29T01:11:00.003+08:00

At 4.442 of Tractatus, Wittgenstein introduces a propositional sign to denote the truth table. For example, if p and q are propositions, then (TTFT)(p, q), or simply (TT-T)(p, q), denotes the proposition with truth table:

p	q	p→q
T	T	T
F	T	T
T	F	F
F	F	T

It is in fact the proposition p→q. Here the order or the truth-possibilities (of p and q) in a scheme is fixed once and for all by a combinatory rule (4.442). Wittgenstein does not state the order explicitly, while the sequences of the truth-possibilities of no more than three propositions (terms of the truth-functions) are shown at 4.31. The order is not stated explicitly because it is really not important in the discussion in Tractatus, except that the truth-possibilities are all F in the last row in the truth table. Thus, (---T)(p,q) is the same as p↓q, where ↓ is the Peirce arrow, and has the following truth table:

p	q	p↓q
T	T	F
F	T	F
T	F	F
F	F	T

For propositions p, q, r, ..., a variable ξ having them as its values (the terms of the truth-function) is used, and the sign becomes (-...-T)(ξ, ...) at 5.5. At 5.501 and 5.502, the sign is further simplified to (-...-T)(`ξ) and N(`ξ) respectively, while the order of the terms (the propositions) is indifferent. The sign is called (5.5) the negation of the propositions. Thus (5.51), if ξ has only one value p, then N(`ξ) is ¬p (not p); if ξ has two values p and q, then N(`ξ) is ¬p∧¬q (neither p nor q). Furthermore (5.52), if ξ has as its values all the values of a function f(x) for all values of x, then N(`ξ) is ¬∃xf(x).

The importance of the Peirce arrow in logic (and therefore the importance of the operation N in Tractatus) is that any logical operation can be expressed in terms of it (completeness). For example,

¬p is equivalent to p↓p;

p∧q is equivalent to (p↓p)↓(q↓q);

p∨q is equivalent to (p↓q)↓(p↓q);

p→q is equivalent to ((p↓q)↓q)↓((p↓q)↓q).

A truth-function can therefore be obtained by successive negation of the propositions. In Wittgenstein's notation, for example, if ξ has values p and q, then N( `ξ) is p↓q. Furthermore, if ξ has value p↓q, then N(`ξ) is p∨q. Any truth-function and any proposition can be obtained by successive application of such operations. Thereby for a series of successive application of an operation O' on a variable a

a, O'a, O'O'a, ...,

Wittgenstein introduces the sign for the general form at 5.2522:

[a, x, O'x]

where the first term of the bracketed expression is the beginning of the series of forms, the second is the form of a term x arbitrarily selected from the series, and the third is the form of the term that immediately follows x in the series. For example if x is the third term O'O'a of the series, then O'x is the fourth term O'O'O'a. Nevertheless, while each of these variables a and x only has one values, the situation of truth-functions or operations on propositions is a bit more complicated, at least in the sense of mathematical rigorousness. Let us consider an expression

[`a, `x, O'`x]

where both a and x have more than one value, and O'`x is a family of operations of different numbers of variables (for a fixed number n, there is a well-defined operation of n variables in this family). The words in Russell's introduction to Tractatus can be used for the explanation of this expression. The symbol means whatever can be obtained by taking any selection of values of a, taking the result of the operation of O' `x on them, then taking any selection of the set of values now obtained, together with any of the originals - and so on indefinitely. Obviously, we cannot interpret it simply with a series as in 5.2522. We cannot regard Wittgenstein's bracket as a symbol in Mathematics or Mathematical Logic.

Hence in Wittgenstein's notation, the general form of truth-function and of proposition (6 of Tractatus) is given by:

[`p, `ξ, N(`ξ)]

where p has all elementary propositions (or just the elementary propositions that appear in the expression) as its values, and ξ has some selection of propositions obtained in the previous operations (together with any of the original elementary propositions) as its values, and N( `ξ) is the negation of all the propositional values of ξ.

"Plato loves Socrates" says that Plato loves Socrates

2008-12-01T00:11:00.001+08:00

To analyse those forms of propositions in psychology such as 'A believes that p is the case' and 'A has the thought p', Wittgenstein says at 5.542 of Tractatus that:

It is clear, however, that 'A believes that p', 'A has the thought p', and 'A says p' are of the form '"p" says p': and this does not involve a correlation of a fact with an object, but rather the correlation of facts by means of the correlation of their objects.

Wittgenstein points exactly against the superficial thought of such forms of propositions - it looks as if the proposition p stood in some kind of relation to an object A (5.541).

First of all, how to understand / explain '"p" says p'? Let us consider a proposition of the form '"q" says p':

"Wittgenstein was taller than Russell" says that Socrates was fatter than Plato.

Everyone, as long as he / she understands this English sentence, can tell immediately without any thoughtful logical analysis that it is a false statement. Furthermore, he / she can also tell immediately that the following proposition is true (no matter whether it is the case that Wittgenstein was really taller than Russell):

"Wittgenstein was taller than Russell" says that Russell was shorter than Wittgenstein.

As Wittgenstein says at 3.14, propositional sign (such as "q") is a fact. Logically, to justify if such propositions of the form '"q" says p' is true, we can examine whether it is the case that q if and only if p, which is just a truth-operation of the p and q as Wittgenstein asserts at 5.54. Besides, to perceive a complex means to perceive that its constituents are related to one another in such and such a way (5.5423). Of course not all propositions of the form '"q" says p' can be understood in this way. Clearly, the following statement is not true:

"If it is raining, then it is raining" says that if it is hot, then it is hot.

Although it is of the form '"q" says p', where q if and only if p, both p and q are in fact tautologies. As Wittgenstein says at 4.461, tautologies say nothing. In the first place, "q" says nothing, so that we cannot even assert the following:

"If it is raining, then it is raining" says that if it is raining, then it is raining.

We cannot assert the following statement either:

"Wattginstein was taller than Sucrotis" says that Sucrotis was shorter than Wattginstein.

As long as the symbols "Wattginstein" and "Sucrotis" do not signify anything, "Wattginstein was taller than Sucrotis" is nonsensical. We cannot compare nonsensical propositions in the first place.

Then how to understand / explain / justify 'A says p'? We have to examine what A said on the objects of the fact p (if A has said something about them) to determine the correlation of this fact (what A said) and the fact p, as Wittgenstein asserts at 5.542. Logically, "p" should say something in the first place, and we compare the fact p with each of the facts represented by the finite set of sensical statements of A. A composite soul would no longer be a soul (5.5421). What about if A says nonsense? We can just tell that A says nonsense, but we cannot have a statement like:

A says that Sucrotis Wattginstein was tham shorter.

It is nothing but just another piece of nonsense! Hence, Wittgenstein says at 5.5422 that

The correct explanation of the form of the proposition, 'A makes the judgement p', must show that it is impossible for a judgement to be a piece of nonsense.

Thus, if I tell you, "I make the judgement that Sucrotis was shorter than Wattginstein," and if I do not tell you a piece of nonsense, it means that I really know two guys called Sucrotis and Wattginstein, and maybe Sucrotis was in fact taller than Wattginstein so that my judgement may be wrong, but at least my judgement is not a piece of nonsense. Besides, propositions occur in such propositions still as bases of truth-operations as Wittgenstein asserts at 5.54.

Nevertheless, Russell says in his introduction to Tractatus that

This problem is simply one of a relation of two facts, namely, the relation between the series of words used by the believer and the fact which makes these words true or false.

But on the other hand, he concludes that the proposition does not occur at all in the same sense in which it occurs in a truth-function. I think that is why Wittgenstein believed that Russell did not really understand the Tractatus.

Anyway, is the heading "proposition" a good example? Is "Plato loves Socrates" really a sensical proposition (it is an example in Russell's introduction to Tractatus)? Is "love" what we cannot speak about? Must we pass "love" over in silence (as the conclusion at the end of Tractatus)? Er... I have such a thought, but my wife doesn't think so!

Identity in Tractatus

2008-11-17T23:49:00.003+08:00

In Tractatus, identity of object Wittgenstein expresses by identity of sign, and not by using a sign for identity (5.53). He speaks roughly that (5.5303):

To say of two things that they are identical is nonsense, and to say of one thing that it is identical with itself is to say nothing at all.

Then he also says at 5.535 that Russell's "Axiom of Infinity", which says that there are infinitely many objects (at 4.1272, Wittgenstein says one even cannot say so), would express itself in language through the existence of infinitely many names with different meanings. In fact, as Russell says in his introduction to Tractatus, the rejection of identity removes one method of speaking of the totality of things.

To Wittgenstein, tautologies also say nothing at all (4.461), although they are not nonsensical (4.46211). Of course, everyone absolutely agrees that tautologies like "if it is raining, then it is raining" really say nothing at all. However, the "problem" of human beings (or of the world) is that the world is so complicated that we cannot determine many tautologies at first sight.

Identity is a relation in Mathematics. There are occasions at which one object may have more than one name / sign - signs are used to refer to the descriptions of some objects, and eventually it is found (proved) that these descriptions have the same reference. In the real world, we might call the murderer of some case of murder X, and eventually we found that he was the man called A: A = X. The introduction of the identity-sign simplifies our deduction.

Negative & Positive Propositions in Tractatus

2008-11-09T16:10:00.000+08:00

As Wittgenstein says at the very beginning of Tractatus that the world is all that is the case. What is the case is a fact. For the totality of facts determines what is the case, and also whatever is not the case (1.12) - if we have the collection of all facts, then we can tell what is not in this collection. Maybe we can call what is not the case a negative fact. A negative fact is just a fact that tells you something that is not the case.

If p is the sign of a proposition that asserts something is the case, then the sign of the corresponding negative proposition can be constructed (by truth-operations) as ~p. However (5.5151), it is also possible to express the negative proposition by means of a negative fact: p is not the case. Functions f(x) like "x is the case", "x is not the case", "x is true", and "x is false" are not truth-functions (in the sense of Tractatus, e.g., 5). But really even in this case the negative proposition is constructed by an indirect use of the positive (5.5151).

Consider a simple example with the notation of Mathematics: 0 > 1. We can simply say that "0 > 1" is not the case. In fact, this negative proposition can also be expressed by 0 ≦ 1. We could define first the sign > for all real numbers, and then just define for any real numbers that a ≦ b whenever "a > b" is not the case if we want such a sign ≦ for the sake of simplicity. Such a sign ≦ is in fact not necessary. Nevertheless, it is clear that "a > b" presupposes the existence of "a ≦ b" and vice versa - if we know when "a" stands to "b" in a certain relation, then we can tell when "a" does not stand to "b" in that relation.

As Wittgenstein concludes at 5.5151, the positive proposition necessarily presupposes the existence of the negative proposition and vice versa.

The General Propositional Form in Wittgenstein's Tractatus

2008-10-26T14:37:00.000+08:00

As it was mentioned in the last article that we use the perceptible sign of a proposition as a projection of a possible situation (3.11) such as spatial relations. Wittgenstein emphasizes at 3.13 that:

A proposition, therefore, does not actually contain its sense, but does contain the possibility of expressing it. ("The content of a proposition" means the content of a proposition that has sense.) A proposition contains the form, but not the content, of its sense.

In addition, the general propositional form is the essence of a proposition (5.471). For all that are possible in logic, Wittgenstein says at 5.473 and 5.4733 that

If a sign is possible, then it is also capable of signifying. Whatever is possible in logic is also permitted. Any possible proposition is legitimately constructed.

He also gives the sentence "Socrates is identical" as an example of a possible proposition. This proposition is, therefore, not only legitimately constructed and permitted, but also capable of signifying. It has no sense just because we have failed to give a meaning to the sign "identical", even if we think that we have done so (5.4733). It is we who failed to use the perceptible sign of this proposition as a projection of a possible situation (3.11).

Relations in Wittgenstein's Tractatus

2008-10-20T01:36:00.000+08:00

Wittgenstein says at 3.1432 of Tractatus that

Instead of, 'The complex sign "aRb" says that a stands to b in the relation R' we ought to put, 'That "a" stands to "b" in a certain relation says that aRb.'

It seems scarcely comprehensible at first sight, especially for those who are familiar with set theory. In mathematics "aRb" really says that a stands to b in the relation R. It is because the world of mathematics is the totality of "such" propositions, while the (real) world is all that is the case as Wittgenstein says at the very beginning of Tractatus. We use the perceptible sign of a proposition (spoken or written, etc.) as a projection of a possible situation (3.11). The distinction is more clear if we consider spatial relation as in 3.1431. All because of such projection, we can say, for example, geometry is a study of spatial relations.

Nevertheless, no matter relations in mathematics or those in Tractatus, they should be well-defined, in the sense that we can tell at the same time what objects are in such relations (which pairs of objects are / are not in such binary relations), and assigns truth values to the objects (to the pairs of objects for binary relations).

Therefore Wittgenstein says at 5.42 that Frege's and Russell's "primitive signs" of logic such as ∨ (disjunction) and → (implication) are not signs for relations. In the first place, such logical signs did not give truth values. A proposition like p→q just tells you something (if p then q), but does not tell you that something (some relation) is true. They belongs to syntax. However, we can actually define relations on all propositions such that, for example, pRq if it is the case that p→q. These relations belongs to semantics.

Besides, Wittgenstein also says at 5.42 that Frege's and Russell's "primitive signs" of logic are not even primitive signs. Primitive signs are names that cannot be analysed further by any definition (3.26), while Frege and Russell even tried to define these logical signs. In fact, "well-definedness" is also a requirement of logical signs in Tractatus (5.46). If a sign is not primitive, we should be able to analyse it further by definition at the same time for all combinations with other well-defined signs (including brackets). If a sign was primitive, we should have introduced (primitive signs cannot be defined) the sense of all combinations with other signs. Therefore, there are no primitive logical sign in Tractatus. The real general primitive signs are the most general form of their combinations (5.46). At 5.461, Wittgenstein further comments that such pseudo-relations of logic need brackets, which is an indication that they are not primitive signs. For example, (p→q)→r and p→(q→r) are 2 different propositions, in which brackets are necessary. More precisely, it is an indication that they alone are "sometimes" even not logical signs, instead of an indication that the logical signs are not primitive. In fact, we can always write (p→q) instead of p→q in Frege's and Russell's notation. In this case, the pair of brackets and → form the logical sign of implication. However, is such a logical sign primitive?

Jan Łukasiewicz (1878 – 1956) has introduced a bracketless notation, in which, for example, Cpq is written instead of p→q. In that case, (p→q)→r and p→(q→r) will be expressed as CCpqr and CpCqr respectively. C is not a sign of relation yet because it does not give any truth value. Nevertheless, because of 5.461, we still wonder if C is a primitive logical sign. It depends.

Many systems of classical logic developed after Wittgenstein's Tractatus really introduced, but not defined, two (primitive) logical signs such as the negation and implication, and then defined all other logical signs such as disjunction, conjunction and biconditional. The syntax of the propositions (formulas) were studied.

Comparing (p→q) with Cpq in Łukasiewicz's notation, we can see immediately that the "aggregation" done by brackets is done by the rule for reading an expression containing the sign C. No matter which notation is used, the logical signs are just used to clarify the meaning. They belongs to syntax. As Wittgenstein concludes at 5.461, signs for logical operations (including those of Łukasiewicz) are just punctuation-marks.

Functions and Operations in Wittgenstein's Tractatus

2008-09-22T00:56:00.001+08:00

Functions in Wittgenstein's Tractatus Logico-Philosophicus do not have exactly the same meaning as those in modern Mathematics. From its context in Tractatus, a function needs not have a well-defined domain (the set of arguments at which the function is well-defined) as in Mathematics, as long as it is logico-syntactical.

If all propositions form a set (the general form is given in 6 of Tractatus), then operations in Tractatus have the same meaning as those in Mathematics -- they are just functions (in the mathematical sense) of one or more arguments (propositions) to the set of all propositions. They are well-defined on all propositions.

As states in 5.251, a function cannot be its own argument because it is not logico-syntactical (3.333), whereas an operation can take one of its own results as its base because an operation is well-defined on all propositions as its bases and produces a proposition as its result.

If all propositions form a set, then what is the meaning of a function of Wittgenstein's with these propositions as arguments? Such a Wittgenstein's function is in fact the value of a mathematical function. In other words, if we denote the function with one argument (for example) by F in Mathematics, then Wittgenstein's function is just F(p) for some proposition p. Wittgenstein calls such F an operation. As states in 5.234, truth-functions of elementary propositions are results of operations with elementary propositions as bases. In Mathematics, we can have a composite function - a function of a function. That's why operations and functions must not be confused with each other (5.25); and a function cannot be its own argument, whereas an operation can take one of its own results as its base.

(Written on 22 September 2008; revised on 12 May 2009)

Wittgenstein's Tractatus Logico-Philosophicus & Russell's Paradox

2008-07-19T16:30:00.001+08:00

"No proposition can make a statement about itself, because a propositional sign cannot be contained in itself," Wittgenstein says at 3.332 of Tractatus Logico-Philosophicus (the decimal figures hereafter refers to the numbers in the Tractatus). He also "disposes" Russell's paradox at 3.333.

Let us consider the following proposition p:

This statement is false.

Clearly, if p is true, then from the content of p, p is false. On the contrary, if p is false, then what p says is true, so that p is true. It is the liar paradox.

At first glance, the problem of the liar paradox is due to self-reference of the proposition. Let us consider a more general version of the paradox. Let P and Q denote the following proposition respectively:

Q is true; P is false.

We see that the paradoxes arise from the actually meaningless definitions of the objects (that is, the propositions) concerned. To define an object, we must use already well-defined terms. A self-referential proposition is meaningless because it is defined by itself. For the last version of the paradox, both P and Q are defined by what are being defined! Let us return to the self-referential liar paradox and rewrite p as

p is false.

Consider a function F(q):

q is false.

Then F(p) is the proposition:

p is false.

Besides, F(F(p)) is the proposition:

F(p) is false.

Here, as mentioned in 3.333, p, F(p) and F(F(p)) are in fact different propositions, and they should not be signified by the same sign such as p (as what liar paradox does), as states in 3.325:

In order to avoid such errors we must make use of a sign-language that excludes them by not using the same sign for different symbols and by not using in a superficially similar way signs that have different modes of signification: that is to say, a sign-language that is governed by logical grammar--by logical syntax. (The conceptual notation of Frege and Russell is such a language, though, it is true, it fails to exclude all mistakes.)

Wittgenstein does not mention the liar paradox but that of Russell. Russell's paradox is a set-theoretical paradox that asks whether the set R defined by

R = {x: x is not an element of x}

is an element of itself. If we use f(x) to denote the proposition

x is not an element of x,

then

R = {x: f(x)}.

However, is the definition of R meaningful? If it was, it had to be able to tell if any given object was an element of R. In particular, could it tell if the object R (if it could be defined) itself was an element? In other words, is f(R) true (or false)? Actually, if R was well-defined, f(R) could be rewritten as f({x: f(x)}), which was also a meaningless self-referential proposition (covered by the definition)!

Russell's solution to his paradox was his theory of types. He arranged all propositions into a hierarchy. The lowest level of the hierarchy consisted of propositions about individuals, not sets. The next lowest level consisted of propositions about sets of individuals. The next lowest level consisted of propositions about sets of sets of individuals, and so on. Thereby the definition (one of these "valid" propositions) of any set only referred to objects of the same "type" (at the same level).

It seems that Russell's theory of types is just an ad hoc solution to the problem of self-reference of his paradox (3.332). However, we see that the rules in his theory are not just syntactic, but semantic - he had to mention the meaning of signs when establishing the rules for them (3.331). On the other hand, Wittgenstein provided a syntactic solution to these paradoxes. More precisely, Wittgenstein did not provide any solution (because a solution is more or less ad hoc), but it is just that there is no such paradox in the system of his Tractatus.