Concrete thinking in its most literal form would be thinking about the stuff that gets poured onto sidewalks. That is something of a joke, but it illustrates the nature of thinking in concrete terms. The idea of “concrete” stands for literal, right now, and immediate, and yet a person who sees the world only in concrete terms would likely have difficulty understanding the abstract nature of its definition. In fact concrete and abstract are often contrasted to each other, where abstract thinking is idea based, able to move to more figurative definitions, and likely to be able to understand conceptual knowledge that exists outside of the moment.
In human development, most individuals begin thinking in concrete ways. This is easily illustrated with infants. If an infant is playing with a toy and the toy is suddenly covered with a blanket, the baby is likely to think the toy is gone: out of sight, out of mind. It takes a while for the baby to realize that the toy is still there if not seen, and this is the beginning of the ability to think in abstract ways.
Still, most children won’t be particularly abstract in their thinking for many years, and will view things in literal ways for a long time to come. As they age, they develop different levels of facility for being abstract thinkers, and some will become very skilled at conceptualizing, while others will retain a more concrete thinking bent.
While most people have the ability to think concretely and abstractly, there are some circumstance where facility to think concretely becomes absent. Certain mental disorders are characterized by a person’s inability to demonstrate concrete thinking and see things from a literal perspective. This is true of any mental illness that causes delusions, such as schizophrenia. At times these delusions create an almost totally abstract world that makes brushes with the concrete rare or difficult. Given medication, many people are able to return to more concrete ways of thinking, but while fully delusional, it can be deeply challenging to interpret a person’s thoughts; they range too far from the concrete.
Similarly, conditions like dementia may cause periods of abstractness, where ability to think in concrete terms is hampered. Alternately, some people are unable to develop abstract thinking due to inadequate brain development or they lose this facility as a result of illness or brain injury. They may remain in a concrete thinking state and be unable to think in abstract terms. There are psychological tests that can gauge ability of abstract and concrete thinking, which might be used to determine certain conditions or treatment. Yet leaning toward a more abstract or concrete thinking style is not, in itself, abnormal.