Jean Piaget, 84, a psychologist whose great sympathy for children and keen observations of how they cope with the world made him a principal founder of modern child psychology, died Tuesday in Geneva, Switzerland. The cause of death was not reported.

In more than 30 volumes, that have been translated into several languages, Dr. Piaget propounded the thesis -- now widely accepted -- that intellectual development is closely related to emotional, physical and social development. The reasoning powers of young children, he said, are limited to concrete objects that they can see, touch or examine through there other senses. Older children, at the ages of about 11 to 15, are able to think in terms of ideas, of abstractions, of logic -- and thus they begin to form a view of the world akin to that of adults.

One of Dr. Piaget's major contributions was his development of a theory that children pass through distinct stages of mental and emotional development. tThis home truth had been observed by other psychologists -- and by numerous parents -- but Dr. Piaget gave it a coherence that has had enormous implications not only for the development of clinical child psychology, but also for education.

The first stage, he said, was the "sensory-motor" stage. Infants begin to learn about objects through their senses.

At the age of about 2, they pass into the "period of concrete operations." In this phase they begin to learn to talk, but they are unable to think beyond the things that they can see and touch. This accounts, for example, for the popularity of the game of "peek-a-boo" among small children -- the persons with whom they play are "not there" when they cover their eyes.

The third stage is the "period of formal operations." It begins at about the age of 11 and continues to 15. At this stage, according to Dr. Piaget, a child "can now reason on hypotheses, and not only on objects. He constructs new operations, operations of propositional logic, and not simply the operations of classes, relations, and numbers."

It is from this point that children begin to think as adults.

Dr. Piaget dismissed efforts to call him an educator, but his work is used widely in the education of small children. According to a long-standing tradition, a child reaches the age of reason at about 6 or 7. But Dr. Piaget has demonstrated that at those ages the reasoning powers of children are limited to things and they cannot work with merely verbal propositions.

A characteristic of Dr. Piaget's research methods was the actual observation of children in classrooms and at their own normal activities. But his theoretical work involved the application of mathematics to his findings -- one of his principal tools in this regard was Boolean algebra.

It was through these and other methods, including interviews of children in quasiclinical settings, that he pursued what he regarded as his main work.This was epistemology, the study of how knowledge is acquired. His work depended primarily on his observations of the actual emotions of children and how they face and solve problems. To Dr. Piaget, finding out why children reach incorrect solutions was as important as seeing how they find the "right" answers. Much of his material was provided by his observation of his own children.

Jean Piaget was born on Aug. 9, 1896, at Neuchatel, Switzerland. His father was a scholar of the Middle Ages. The boy became interested in philosophy and biology and by the age of 15 had published studies of mollusks found in the Alps.

His studies persuaded him that nothing in nature could be understood except in terms of "totalities" or "structures of the whole." Believing that this was particularly true of intellectual processes, he turned to psychology. In Geneva, he studied with Carl Gustav Jung, the eminent student of Sigmund Freud. Piaget earned a doctorate at the University of Neuchatel in 1918 and spent the next three years studying in Paris.

In 1921, he returned to Geneva and began his long association with the Jean Jacques Rousseau Institute and the University of Geneva. Although he never received a degree in psychology, he was professor of child psychology at the University of Geneva from 1929 to 1971, when he was accorded the status of honorary professor. Over the years, he also taught at numerous other universities, including the Sorbonne.

In 1955, with the help of the Rockefeller Foundation, he established the International Center for Genetic Epistemology and served as its director from the time until his death.

Among Dr. Piaget's better known books are "The Language and Thought of the Child," "The Judgment and Reason in the Child," "The Origin of Intelligence in the Child" and "The Child's Construction of Reality."

He received honorary degrees from more than 30 colleges and universities, including Harvard, Yale, the Sorbonne and The University of Moscow.

Dr. Piaget, a cheerful, pipe-smoking man given to riding a bicycle and wearing a blue beret, remained active until near the end of his life. In the winters, he would work with his assistants. In the summers, he would retreat into the Alps to think and to work on his books.

A widower, Dr. Piaget is survived by three children.