True knowledge–maybe understanding would be better–comes from moving from particulars to universals. 

We can’t simply jump to the universal, because universals are always and only instantiated in particulars. Tree doesn’t exist in the world independent of the trees themselves.  We thus come to know Tree through trees.

Since the ultimate ground of universals is the mind of God–they are ideas therein–the movement from particulars to universals requires an inward turn.  Why? Because knowledge of God comes from knowledge of God’s fullest instantiation in the world, in His image, i.e. human nature.  (Hence why knowledge of God and understanding of creation becomes more and more possible the more we conform our nature to Christ, the perfect image). 

Universals contain within themselves the richness of the particular.  There is not any aspect of any tree that is not contained within the Tree.  (An aside, we glimpse in this the mind-shattering beauty of the fundaments of reality, think Charles Williams, The Place of the Lion). 

The process of turning inwards and moving from particulars to universals is superficially easy to compare with abstraction.  We do in a sense abstract the form of a thing from its particular instantiation in order to understand it.  However, abstraction, in the more conventional sense, is in actuality a movement away from true understanding.  Abstraction denudes particulars of their richness, rather than uniting that diversity in unity.  Compare an actual tree to a generic tree on an architectural diagram. 

Abstractions are constructions of the mind.  As constructions they are necessarily inferior to and have less actuality than their template.  Abstractions are less real than the things from which they abstract from.  In a sense, not real at all. They are thus mere reflections or shadows of the particular, a step down the chain of understanding, further into the cave. 

Mistaking abstractions for universals is one of the great and characteristics errors of our time.