The ancient Greek philosopher Heraclitus once said, "Everything is in constant flux. No one can step into the same river twice, because neither the river nor the person is the same."
The German philosopher Husserl used sound as an example. Sounds rose and fell, varying in intensity. An inner perception unified the sounds one heard with one's direct grasp of time, then projected them onto the "river." Within those projections, the same sound continually faded, then renewed itself with new sounds, reproducing itself through the continuity of passing time.
It repeated itself again and again, eventually forming a river of lingering echoes. Even after the bells had stopped ringing for quite some time, people still felt that the echoes lingered in the air.
Normally, such a phenomenon would not last long. Yet the "afterglow" Harry had left behind not only remained for a long time, but was also remarkably intense. The Centaurs who first noticed it mentioned the low temperature there. Expecto Patronum would not cause a drop in temperature, but Dementors certainly could.
Harry, who had caused all this, had already gone home for the summer holiday. Even if someone asked him about it, he probably could not explain why. At the time, he had only one thought: save Sirius and himself, and make certain that his father would not come.
The stream of consciousness was created by the mind. It too was a constantly changing river that never stood still. Pavlov regarded consciousness and behavior as "reflexes," requiring external conditions to trigger regular responses, but no one would think Expecto Patronum could be cast through a "conditioned reflex."
Perception was given, much like the salary and care a "Patronus" gave its "retainer." It was generally believed that the caster, as the retainer, had to repay the Patronus with