Again, “farther up and further in” suggests a constant state of spiritual ascendency, as the characters grow closer and closer to Aslan. Digory compares Aslan’s eternal realm to Plato’s philosophy because Plato had a theory of Forms, which held that everything in the physical world was merely a proxy for its ideal form, which existed in a different plane. Here, the characters are getting to witness the ideal version of Narnia, an already fantastical land, which is glorious beyond their comprehension.