I suspect greatness is closely related to a feeling, a feeling of deepness that one gets when whatever is experienced or felt resonates within those deeper, more ancient brain structures.
But it also has a social dimension. I could personally think something is great that no one else really talks about, and it very well could be, but it is a different feeling than when a large body of individuals each has the same feeling and agrees about its source. So our concept of greatness seems to do with activation, when a great many connections are activated across a large scale of activated individuals. Our feeling of greatness is influenced both by the feeling within us, and the apprehension of that feeling being spread out across the land.
There is something else about greatness, akin to the dancing surface of a deep ocean whose beauty is made great by the presence of a deep dark void dropping away underneath. That which is illuminated and perceivable is amplified by that which is inscrutable underneath. In the same way, our experience of some symbolic truth such as in literature or film, or a sensuous truth such as within a work of art or a musical composition, has underneath it some vast expanse which we will never understand.
