Aristotle... wasn't that this position?
Thats a good question that we will have to one day figure out. Do you have a reference that you were thinking of, or anything in particular?
The only thing that I am aware of that may be applicable is that supposedly Aristotle transferred Plato's "ideas" from an external existence I may realm beyond the reach of the senses to an "intrinsic" or "essentialist" existence inside the object under consideration (things that appear yellow have an essence of yellow in them).
But I could not provide you with a good cite to that either, so unfortunately I am not in position to clarify the question.