Just a small theory I've been pondering lately and I wanted to get some feedback on it. Generally, I'd say that most consider the sixth sense to be interacting with worlds other than this one, but the more I think about it, the less sense that idea makes.
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In my opinion, ESP is not the sixth sense. Empathy is.
ESP abilities are simply extensions of the regular five senses. Like clairvoyance is an extension of sight. The other reason behind my theory is clairsentience, the ability to sense the spirit world. It really has no corresponding regular sense, but I think empathy fills the void quite nicely.