Speakpigeon wrote: ↑Tue Feb 26, 2019 11:39 am
Logik wrote: ↑Mon Feb 25, 2019 12:25 pm
So define "=" properly then! What are you waiting for??????
Define "=" so that the following ENGLISH statements evaluate correctly.
John is human (A = C => True)
Jane is human (B = C => True)
John is Jane (A = B => False)
No problem.
First, you've mixed up two different formalisms so we need to unpack the argument to try to make sense of it.
First, the argument with "=". It is valid:
A=C;
B=C;
Therefore A=B.
No problem.
Second, the argument with "is":
John is human;
Jane is human;
Therefore, John is Jane.
It is ambiguous. So, to make sense of it, you can try different interpretations.
First, "is" interpreted as "∈":
John ∈ Human;
Jane ∈ Human;
Therefore, John ∈ Jane.
The conclusion may be true but the argument is not valid because of the undistributed middle. So, the conclusion, even if true, doesn't follow from the premises.
Second, you could interpret "is" as meaning "=", like this:
John = Human;
Jane = Human;
Therefore, John = Jane.
This is valid.
You can also interpret "is" in two different ways in the same argument, like this:
John ∈ Human;
Jane ∈ Human;
Therefore, John = Jane.
The conclusion may be true but the argument is not valid because of the undistributed middle. So, the conclusion, even if true, doesn't follow from the premises.
This last interpretation is what normal people with a brain do without even thinking about it when they come across this argument.
Tell me if you see a problem here.
EB