The apparent completeness of the mechanical theory of the universe satisfied the inquiring intellect. The excitement caused by the scientific discussions and discoveries from the time of Heraclitus subsided. But after a short intervening period, when public attention had been largely centered on practical affairs, there was a reaction against science. When scientific principles were quoted a tendency was shown to question their validity and usefulness. This resulted in inquiries into the sources of knowledge and conduct and ushered in a new intellectual era that is now known as the Humanistic period which, beginning about 450 B. C., extended to 400 B. C. The Sophists, who were teachers of rhetoric and were accustomed to studying the phrasing of verbal statements, became active in searching for the foundations of thought. The Protagorean theory of knowledge was based on Empedocles's doctrine that the inner atoms advance to meet the outer ones. Perception is the resultant product of these atoms when they collide. They believed that this perception is something else than the perceiving subject and is also something different from the object giving birth to the perception. It is conditioned by both, but has a distinct existence. The doctrine of the subjectivity of sense perception was developed in explanation of this psychological problem. From this it followed that knowledge must be strictly personal and could be true only under conditions existing at the instant of perception. These limitations caused Protagoras to advance his theory of relativity, which teaches that man is the measure of all things. Facts are what appear to each individual to be statements of truth. Isocrates, Plato, Aristotle, and Socrates were the leaders of this intellectual movement. Socrates developed the Pythagorean theory of intelligible forms. The specific qualities of the senses belong to the realm of perception. When these are withdrawn from an object of thought there remains only the form or idea. Therefore it is evident that pure, intelligible forms constitute the essences of things. The early scientists, such as Democritus, thought, perhaps, in terms of atom forms. Socrates, Plato, and later teachers looked upon forms as conceptions of similar logical elements. Knowledge, in the view of Democritus, was essentially rationalistic. Plato considered knowledge as having ethical and æsthetic purposes within itself. Each of these types of rationalism stimulated Greek thought and resulted in a strong impulse to philosophical and scientific investigation. They prepared the outlook for Aristotle.