Well? You know, one so old that they've developed their weaker functions and don't appear strongly either way.
I have one that could be either, but they grew up in a fiercely-intellectually-competitive house under an INTJ, so I can't tell how much of their book habit that dates to their childhood is related to nature or nurture.
Their place is utter freaking chaos like you would find with either - dirty, books laying everywhere, cans/refuse all over, etc, etc, etc. I've gone on trips with them and they do the same madness in hotel rooms, which the INTPs I've worked with were well known for. It bothers me somewhat, but not enough to do anything about it.
Anyhow, he wants to join me for a leg of my extreme points trip this summer and I'm trying to figure out what he actually is.