Abstract concepts include freedom, good and evil, love, feminism, success, morality, and chauvinism. Abstract concepts are those for which there is no physical referent; concepts with physical referents are known as concrete concepts.
Philosophers are often interested in abstract concepts because they raise issues of ontology and sensory experience. Some philosophers, beginning with Plato, have argued that abstract concepts are the primary subject to be studied in philosophy or metaphysics.
The issue of abstract versus concrete thinking is a relevant one when studying children's mental and psychological development. Young children are not able to think abstractly. For instance, as the Brain Injury Association of New York State points out, a 2- or 3-year-old child who reads Dr. Seuss' book "Green Eggs and Ham" is typically only able to understand the story concretely as that of a person who does not want to eat green eggs and ham and perhaps may understand that it is about someone changing his mind. On an abstract level, the story is about the abstract idea that people can change the way they think or feel even when they do not think they can. A child can come to understand this abstract thought by having an adult discuss it with him, thus moving the child closer to the ability to understand abstract concepts.