Skepdick wrote: ↑Sat Oct 05, 2024 10:12 pm
wtf wrote: ↑Fri Oct 04, 2024 8:39 pm
Then I define an odd number as an integer that's not even.
The semantics aside - this step is not constructively valid.
Negation is involutive in a classical setting because Excluded Middle. It's not involutive in a constructive setting.
For this particular instance you need to prove that odd and even are fully exhaustive categories first before you can define an involutive relaton between the "odd" and "even" predicates.
I'd like to understand your reasoning more clearly.
Suppose I make the following definition:
An integer is even if it's divisible by 2.
Here "divisible by 2" has its obvious meaning. The integer is 0 mod 2. Or it has remainder 0 upon integer-division by 2. I hope we don't have to play word games about the meaning of divisible by 2.
Suppose I make the additional definition:
An integer is odd if either
a) It's not divisible by 2; or
b) Its divisible-by-2 status is in some kind of intederminate state.
I have two questions:
1) What's wrong with that definition? A number is odd if it's not even; namely, if it's either definitely not divisible by 2, or else it's in some kind of intuitionistic twilight state.
So I have an "Even machine." I enter a number. If it's divisible by 2, the machine outputs "Even." If it's definitely not divisible by 2, or if the machine can't figure out if it's divisible by 2, it outputs "Odd."
What's wrong with that?
2) I can't believe Brouwer and all the other clever early intuitionists couldn't define an even number. Convince me that this is the correct interpetation, and that you really think I can't define what an even number is without the law of the excluded middle. Can you explain to me how determining if an integer is even could end up in an indeterminate state?
After all, dividing an integer by 2 can be completed by a Turing machine in a finite number of steps. So we can always have a Yes/No machine for the question, "Is n even?" So I just don't understand what an indeterminate answer could be in this context.