External cause: it is the external condition for the existence and development of things. It acts on the existence and development of things through internal factors, accelerating or delaying the development process of things, but it cannot change the fundamental nature and basic direction of things' development. So the internal cause is the first reason, and the external cause is the second reason.
The most classic example is the chick hatched in the egg. Eggs can hatch chicks, while duck eggs can only hatch ducklings because of different internal factors. This is called "internal cause is the foundation of the existence of things, the inherent essence that distinguishes things from other things, the source and motive force of the movement of things, and it stipulates the basic trend of the movement and development of things". However, not all eggs can hatch chickens. Eggs can only hatch at proper temperature and humidity. If the conditions are not met, the eggs cannot hatch. Moreover, no matter what conditions, eggs can't hatch ducklings. As the saying goes, "external factors are external conditions for the existence and development of things, but they can't change the fundamental nature and development direction of things". To sum up, eggs must meet two conditions when they hatch into chickens: 1, and eggs are eggs, not other eggs or stones. 2. Appropriate temperature and humidity. This is called "internal and external factors coexist in the development of things, and both are indispensable". .