developmental processes. Despite being the first to develop a systemic study of cognitive development, Piaget was not the first to theorize about cognitive development. Jean-Jacques Rousseau wrote Emile, or On Education in 1762. He discusses childhood development as happening in three stages. In the first stage, up to age 12, the child is guided by their emotions and impulses. In the second stage, ages 12–16, the child's reason starts to develop. In the third and final stage, age 16 and up, the child develops into an adult. James Sully wrote several books on childhood development, including Studies of Childhood in 1895 and Children's Ways in 1897. He used a detailed observational study method with the children. Contemporary research in child development actually repeats observations and observational methods summarized by Sully in Studies of Childhood, such as the mirror technique. Sigmund Freud developed the theory of psychosexual development, which indicates children must pass through several stages as they develop their cognitive skills. Maria Montessori began her career working with mentally disabled children in 1897, then conducted observation and experimental research in elementary schools. She wrote The Discovery of the Child in 1950 which developed the Montessori method of education. She discussed four planes of development: birth to 6 years, 6 to 12, 12 to 18, and 18 to 24. The Montessori method now has three developmentally-meaningful age groups: 2–2.5 years, 2.5–6, and 6–12. She was working on human behavior in older children but only published lecture notes on the subject. Arnold Gesell was the creator of the maturational theory of development. Gesell said that development occurs due to biological hereditary features such as genetics and children will reach developmental milestones when they are ready to do so in a predictable sequence. Because of his theory of development, he devised a developmental scale that is used today called the Gesell Developmental Schedule (GDS) that provides parents, teachers, doctors, and other pertinent people with an overview of where an infant or child falls on the developmental spectrum. Erik Erikson was a neo-Freudian who focused on how children develop personality and identity. Although a contemporary of Freud, there is a larger focus on social experiences that occur across the lifespan, as opposed to childhood exclusively, that contribute to how personality and identity emerge. His framework uses eight systematic stages that all children must pass through. Urie Bronfenbrenner devised the ecological systems theory, which identifies