One of these theorists is Piaget. Piaget believed that children were active thinkers and that their minds developed through a series of universal and irreversible stages, from simple reflexes to abstract reasoning. During these stages, Piaget theorized that children's maturing brains build schemas that are used and adapted through assimilation and adaptation. The first of these stages theorized by Piaget is the sensorimotor stage. This stage, which lasts from birth to almost 2 years of age, is when children learn about the world primarily in terms of sensory impressions and motor activities. Children learn through adaptation, assimilation and accommodation. During this period of rapid development, infants also acquire object permanence, the awareness that things continue to exist even when they are not perceived. The preoperative phase occurs from 2 to 7 years of age. At this stage, children learn to use language but are unable to execute complex logic. It is during this period that children develop a sense of self-centeredness and also a theory of mind, or the ability to read the mental state of others. Once a child matures to around age 7, he or she enters a concrete operational phase that remains until around age 11 or 12. At this stage, children acquire the ability to think logically about concrete events. Piaget believed that during the concrete operational phase children become capable of