PeteOlcott wrote: ↑Tue Mar 26, 2019 4:53 pm
I speak of Truth generically as applied to both formal and natural language.
That would be a mistake then. "Truth" has a number of different and incompatible conceptions.
Formal languages are closed systems.
Natural languages are open systems.
To pretend they are the same beast requires ignoring their differences. One is just an abstract system for manipulating symbols, the other is 'the way people use language' e.g utility.
PeteOlcott wrote: ↑Tue Mar 26, 2019 4:53 pm
Truth is ultimately anchored in a set of expressions of language that are defined to be true.
Defined by whom and for what purpose?
PeteOlcott wrote: ↑Tue Mar 26, 2019 4:53 pm
If not then how do you prove that a dog is not a type of cat?
I don't. By definition a dog is one type of thing. A cat is another type of thing.
If they were the same type of thing I would not have defined two types.
PeteOlcott wrote: ↑Tue Mar 26, 2019 4:53 pm
Dogs and cats are defined axiomatically to have a set of properties.
First they are recognized/conceptualized as different things.
Then they are defined as different things.
BUT, I could recognize/conceptualize cats and dogs differently. I could conceptualize them as types of types and call them animals.
And I can go ahead and define what properties an "animal" has.
In the object-oriented programming paradigm this is known as multiple inheritance:
https://en.wikipedia.org/wiki/Multiple_inheritance
PeteOlcott wrote: ↑Tue Mar 26, 2019 4:53 pm
Some of these
properties are in common and some are mutually exclusive. The mutually exclusive
properties of cats and dogs are the axioms from which the theorem {cats are not dogs}
can be inferred.
You are still alternating between two paradigms: axiom/theorem (Mathematics) and decidable/undecidable (Computation).
Conversely from the above, the common properties of cats and dogs are the axioms from which the theorem {cats}, {dogs} ∈ {Animals}
I don't need to infer the theorem "cats are not dogs" because that's backwards thinking.
I look at the world. I see two things. I recognize that in some ways they are different and in some ways they are the same.
If I DECIDE that they are different things then I will form two conceptions/categories and define two types: Cats and Dogs.
And so I could say "Here is a cat and here is a dog".
If I DECIDE they are the same thing then I will form one conception/category and define one type: Animals
And so I could say "Here are two animals".
Depending on the context, purpose of conversation and intentions of the interlocutors I may want to USE either taxonomy.
So I can DECIDE either way.
This is Binary classification 101 stuff:
https://en.wikipedia.org/wiki/Binary_classification
It is at the core of the concept of "precision".
1 bit of information = 1 distinction.
Animals => 1 bit of information
Cat-animals and Dog-Animals -> 2 bits of information.
Methinks you are going too far in what logic/truth is and what it's for. I am on the same page as Scott Aronson (
https://www.scottaaronson.com/blog/?p=710 ) the Turing machine is at the foundation of human thought. What we have is a formal model of the human mind.
Logic is simply a tool for executing algorithms in one's mind.
Natural language is for communicating between minds.