In the next place, our atomic make-up involves the presence of other atomic complexes, if we are to have any psychical activity. External things must stimulate us. But these external things are atoms in action. They can, however, influence us only by coming into contact with our bodies. Only by impact on our bodies can they set in motion the fire atoms which are scattered through our bodies. Every kind of knowledge or mental life involves the participation of the fire atoms in us. Thus mental activity involves two factors; the fire atoms within us and an external group of atoms without us. How did Democritus explain the varied mental life as the resultant of these two factors? He employed the theory of effluxes, belief in which he shared with his time. This is a purely physiological assumption, originated by such Cosmologists as Empedocles, that somehow external bodies send off emanations from themselves which strike upon our bodies. Most objects in the world influence us at a distance and only through the emission of these effluxes. Democritus conceived these emanations to be little copies or “eidola” of the thing that sends them off. To illustrate Democritus’ meaning: a tree is seen by me because little trees, thrown off by it, hit my eye. This theory retained its position in philosophical circles until after Locke. It persists in the popular mind to-day. It is a general belief that a thought is a copy, photograph, or image of the thing. The words “image” and “imagination” betray their origin. It was believed by Democritus that such copies set in motion the sense organs and through them the fire atoms. The effluxes can, however, affect only those organs of the body that have similar formation and similar atomic motions.