"People are naturaly good" is one of the main cornerstones of the philosophy of Mencius.
Although this is often taken as a cornerstone of Confucian philosophy in general, not all Confucians believe wholeheartedly in Mencius's optimistic assesment of human nature.
I would like to give a detailed proof from Mencius as to why people are good, but Mencius being both a very early philosopher, and a Chinese philosopher, didn't waste that much time on trying to prove this dictum. It was somewhat of an axiom, not specifically needing proof.
The one argument that Mencius does give to explain his reasons for this belief is that everyone, when seeing a baby crawling into a well, would react with horror. Although there are some who would argue that this is not always true, or not neccesarily true, such arguments deny the immediate knowledge of the heart.
In another episode, a philosopher from the legalist school insisted that people are like water, they flow wherever the slope takes them, towards good or towards evil. Mencius replied by working the analogy: Water always flows down. Hitting the water with an oar may make a big splash, but that doesn't tell us about the water, that tells us about the oar, or perhaps about the person hitting the water. Human nature flows towards goodness just as much as water flows downhill.
Whatever the inherent truth of Mencius' statement was, it has certainly produced a fair amount of good in the history of Zhongguo. Whatever arguments can be made about the decadent ruling class, or about the stagnation of Chinese society, China may be the only country that believed that it could keep the peace most effectively by educating people and letting them live their lives in tune with the seasons and their own development.
Also, I think the fact that a Chinese philosopher used obtuse bizarre arguments like "chilren falling into a well is bad" to prove that "people are good" certainly makes such Western philosophers as Plato, with their belief in a cave reality that can only be transcended with group marriages and geometic shapes seem even more sane, rational and normal then they are.