What qualifies something as a "sense"?

I'm not going to be so weird and philosophical as last time, don't worry. I was just thinking about picking up on emotions, the way it feels, and the language we use to describe it. Sure, we pick up on little clues through all of our normal senses that let us know how people are feeling, but that's not how we really think about it. When someone is really happy, or really angry, we say that it's palpable. We say tension is so thick you could cut it with a knife. When it's spring, we say love is in the air. We don't actually think, "Certain ways they are speaking and acting and looking tell me that they are angry/happy/whatever, so now I know that." We "feel" it at an unconscious level

So no matter how we may get the clues that give us perceptions of people's emotions, we have this whole system in our brains that automatically interprets that information, and feeds it into our conscious through a channel that's totally separate from our other senses. So to some extent, I think it's fair to call it another sense. And I, for one, think that's pretty cool.