The individual acts only if he experiences a need, i.e., if the equilibrium between the environment and the organism is momentarily upset, and action… - Jean Piaget

Piaget is also concerned with attempts of Maine de Biran, Bergson, Sartre, and Merleau-Ponty to construct a philosophical psychology as opposed to a scientific empirical psychology. He believes that the difference between philosophical psychology and scientific psychology lies neither in the fact that the former concerns itself with “essences” (Husserl), with “irrationality” (Sartre), nor in its use of introspection. He sees the difference as being one rather of method: philosophical psychology neglects objective verification and grounds itself in subjectivity, although claiming to arrive at objective knowledge through intuition.

Formal logic, or logistics, is simply the axiomatics of states of equilibrium of thought, and the positive science corresponding to this axiomatics is none other than the psychology of thought.

In fact, objects are known only through the subject, while the subject can know himself or her- self only by acting on objects materially and mentally. Indeed, if objects are innumerable and science indefinitely diverse, all knowledge of the subject brings us back to psychology, the science of the subject and the subject's actions.