A concept like “Eternity” cannot be verified empirically. It is just a possibility that we can infer, a construct of the mind. Pure mathematical entities like squares, triangles or circles are also mind-constructs, but these can be somehow represented and replicated as concrete objects in space. How could we represent “eternity”, in a drawing for example? It cannot be even represented. One could think of a circle, because it is a curved line with no beginning and no end. But at the same time, a circle is a limit, and it has a dimension. So, it embodies just one aspect of what we can grasp as “eternity”.
In this sense, I think that the potential degree of representation of abstract concepts is variable, and it depends both in the nature of the concept and the medium chosen. If I ask my architecture students to represent in drawings concepts like limit, sequence, rhythm, enclosure, dynamism, horizontal, vertical, the results may vary, but probably they will share some common features, and some degree of objective universality. But what about happiness, or justice? These are a completely different kind of concepts, which do not have an immediate and intuitive form of representation. If we contextualize this distinction in the ambit of architecture, the problem becomes even more patent. The matter of architecture is, a priori, non-representational and non-referential. Painting, music and sculpture are more adequate mediums for representation, in this sense. Can we conceive a building as if it was a painting, trying to "express" concepts as in a blank canvas?