ext_15881 ([identity profile] neoanjou.livejournal.com) wrote in [personal profile] pozorvlak 2011-06-21 06:12 am (UTC)

"if X then Y" is equivalent to its contrapositive

I've always had a little problem with this - largely because I think I was introduced to it badly by the statement 'All Ravens are black' => 'All not-Ravens are not-black', which strikes me as being false, as it implies that blackbirds (a black not-raven) cannot exist.

I understand that this is a fun post, and all a bit tongue-in-cheek, but isn't a true counter-positive to Rule 34 as stated:

"If it does not exist, there is not porn of it on the Internet"

Which could be construed as also false, depending on your definition of 'exists'.

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