I think that mathematics is primarily based on substitution where we replace a set of symbols with another set of symbols which are equal or equivalent in some cases. How do we decide that ? By "meaning" l meant the criterion for substituting one expression to another. — Wittgenstein
If there is a match between a rewrite rule and an expression, you can rewrite the expression. So, it all depends on the rewrite rules of the system. These rewrite rules have no particular "meaning".
Say that there exists the following rewrite rule in the system: kk* --> k+, with k any arbitrary subexpression, then we can rewite the expression xyabc(abc)*rs --> xy(abc)+rs. This has no "meaning". The resulting expression is just the result of the mechanical application of the rewrite rule on the original expression.
Formalism has axioms and there are rules of inference etc. It cannot work without them. — Wittgenstein
Both axioms and rewrite rules are arbitrary. For example, the
SKI combinator calculus uses the following rewrite rules:
Ix --> x. Kxy --> x. Sxyz --> xz(yz).
We can use this rewrite system to rewrite input expressions to output expressions, and derive new theorems from the system. For example, since SKxy -> y, we can see that SKx = I for any arbitrary choice of x. This theorem is meaningless, because the statement, which is provable from the construction logic of the SKI system, that "∀x in D: SKx = I" does not correspond to anything in the real, physical world.
It is a single character but can we substitute it with numbers ? — Wittgenstein
Infinity itself is a Platonic abstraction that is compatible with numbers, which are themselves also Platonic abstractions. Numbers are themselves no real-world objects either. Infinity is compatible when you can extend the rules for arithmetic to support the inclusion of infinity. while not damaging the algebraic structure.
Consider the real line, all the real number lie on it but infinity doesn't. — Wittgenstein
We don't care about lines in this context.
Therefore by allowing infinity, we sort of compromise the formal system. This is the basic idea behind the constructivist approach, if l am wrong, you are more than welcome to correct me. — Wittgenstein
I think that you correctly depict the constructivist view on infinity. But then again, I don't read much constructivist literature, because in my opinion, they are missing the point anyway. Cantor's elaboration of the concept of infinity is nicely consistent. I have no problem with it. I really do not see what the fuss is all about.
I think that when we introduce the infinity symbol, we will have to drop associative law and commutative law too. — Wittgenstein
Concerning commutativity, the problem does not seem to occur in x + ∞ versus ∞ + x, as both expressions get rewritten to ∞. In my impression, the arithmetic rewrite rules handle the symbol perfectly well. Adding ∞ should not modify the algebraic structure. Otherwise, you should not add it to the domain that you are dealing with. So, for example, if D' extends D with ∞, and <D,+> is a group, then <D',+> should also be a group. Otherwise, don't bother adding it.
By introducing the symbol into the rules and not being able to generate it from the real numbers is cheating. Is this extension valid ? — Wittgenstein
If adding it, can be done while preserving algebraic structure and therefore consistency, you can go ahead and add it. It does not even need to be about numbers.
In fact, there are situations where you must add ∞ in order to guarantee the consistency of field operations. For example:
For current cryptographic purposes, an elliptic curve is a plane curve over a finite field (rather than the real numbers) which consists of the points satisfying the equation y² = x³ + ax + b along with a distinguished point at infinity, denoted ∞. This set together with the group operation of elliptic curves is an abelian group, with the point at infinity as an identity element.
Without identity element, point addition would not be a group operation. The domain here does not consist of numbers but of two-tuples (x,y):
The equations (for point addition) are correct when neither point is the point at infinity, (0,0). When adding the point at infinity to another point, the result is simply the other point.
Elliptic curve arithmetic has obviously nothing to do with the real, physical world. It was not abstracted away from the real, physical world. Elliptic curve arithmetic is a Platonic abstraction that has characteristics and properties that turn out to be interesting, while adding a point at infinity is not only a requirement for consistency, but it also happens to work absolutely fine.