We can think of these four different ways as a pie. If one of the pieces is missing, the pie is not complete. It's the same with children. If one part of the child is not developed, the child does not grow into a healthy, happy adult. He or she is not complete.
Physical development is the way a child's body grows.
It is normal for children to grow at different rates. For example, some might walk at 11 months and some might walk at 16 months. Intellectual development involves a child's thinking. Children learn to think by undertaking challenging tasks. Reasoning their way through different tasks helps children learn.