Ayn Rand covers this in her book "Introduction to Objectivist Epistemology". In her theory, focus (or selective attention) is the essence of free-will (the ability to direct the object of our consciousness). Focus also (not surprisingly) plays a crucial role in the formation of concepts (or classes), the main theme of the book. She describes intelligence as the capacity to deal with wide abstractions and long conceptual chains (my paraphrase).