If

[Little Rik]

this has nothing to do with the fact that the third eye is there.

How do we know this?

I would say........this is a claim for you and a reality for me.
Yogi are always innovative while non yogi rely on what they can perceive mainly with body-mind.
Yogi after realizing that peace of mind can not be found anywhere in the material-physical-mental side of ourselves they come to the conclusion that it can only be found in the spiritual side so how can be found then?
Again yogi study how the body-mind works.
Glands in the body control a specific part of body or mind and along the spinal cord there are the most important glands with the pineal gland at the very top.
The pituitary gland control the mind but over the mind there is something else and this is not something physical-mental.
How do we know this?
We know this by practicing and cleaning up the dirt that prevent the gland to operate in a perfect way.
As the clouds prevent the sun to shine also the dirt prevent the pineal gland to let the divine flow to shine.
It is by practicing that we can understand how the system works.
The same thing is valid for everything else.
The practice is everything while the theory is nothing and the issue will stay as a claim for ever or until one decide to do something about it.