Posted: Apr 14, 2012 11:04 pm
by lobawad
Little Idiot wrote:
asdfjkl wrote:ok my idea that since it's self evident that self evident things exist it is also self evident that non-self evident things don't exist.


Thats simply a logical error.

If every A is B, then every not-A is not B may seem fair enough on first glance.
in asdfjkl's case A= self evident thing, B= exist.

But that cant work with exist, because exist is not a category of things which excludes other things - or in other words, all things exist, because things in this context means 'things which exist'.


Nope, that does not work because "all not-A are not-B" is never the logical equivalent of "all A are B".

All Frenchmen are human.
All non-Frenchmen are non-human. < bzzzzzz

The logical equivalent of "all A are B" is:
all non-B are non-A

All Frenchmen are humans.
All non-humans are non-Frenchmen


Every cat is feline, dogs are not cats, therefore no dogs are feline. - Thats fine.


Tisn't fine, see above. The logical structure there is the same as:

All cats are animals, dogs are not cats, therefore no dogs are animals.

Not fine.


And its fine because feline is defined as being 'only cats'.


Nope. What you're looking for here would be:

All non-cats are not felines.
Dogs are non-cats.
Therefore dogs are not felines.

(I don't know what a biologist would say about cats and felines, we're talking about the logic here.)


Every cat exists, dogs are not cats, therefore no dogs exist. - Thats clearly rubbish. And its rubbish because exists is not defined as being 'only cats'.


You're mixing up validity and soundness, at the least. It's rubbish because ~A > ~B is not the logical equivalent of A > B.