Why there are Mermaids.

Some people believe in mermaids as a type of water elemental,and that they exist on the astral plain,but not the physical plain

What's important to realize and consider in all things astral is that much of what we view things as is a subconscious interpretation.

In the case of mermaids being a variety of water elemental, it is usually because that particular spirit possesses qualities that might subconsciously connect with those qualities you expect a mermaid to have. As such, since there are no true visual sensory organs that view astral energy, your mind translates the energies into the image of a mermaid to help you understand that which you encounter.

So astral mermaids? Sure. If that's what you want to call them. Physical mermaid? First reported by a bunch of vitamin-d (and all other nutrient) deficient sailors who may well have been drinking dirty water supplemented heavily by alcohol... Which in turn might have been mixed with gunpowder to create "grog"... well.. I think we're getting the picture.

could just the word mermaid be part of a spell.to rid the body of madness.or something else.

If the word provides you with symbolism that might aid in such, why not?

Well There are 4 natural elements not including Spirit and the 4 Elemental beings for them are: Earth:Gnome, Air:Faerie or Sylphs, Fire:Dragon, Water:Mermaid or Undine. Personally I belive in these elementals to be spirits who if you bond a friendship with them they if choosen ma appear to you.

Honestly, that is a somewhat watered down assessment. Most of those entities you described are faeries, or otherwise part of the Fair Folk mythology, making the idea that all faeries are related to the element of air somewhat off. The Fair Folk in general are party to a wide range of elements, as one might discover in the way that you connected several of them to specific, different, elements. And those who practice dragon magic would likely be quick to point out that dragons often favor a wide range of elements, and may not even choose one to favor in particular.

As previously suggested (oh how I wish people would read AND comprehend) elementals are spirits that appear to us in these various forms. But the friendship part? You're spot on with that.

There is another theory but, I think both of them might go together. The theory is that sailors when they were on their boats would see the backs of manatees and think they were mermaids. In my opinion I think the lack of vitamin D attributed to that because it would have affected their vision, and so on and so fourth.

So, basically, sailors were, for many reasons, completely insane and not to be trusted. (I still don't trust them lol).

My paw paw is was a sailor and so was my great (many greats) paw paw (a pirate too :S). I only trusthim half of the time. <.<

There is a similar type of Fae to mermaids :) but they very rerly choose to appear