Natural causes are physical forces like gravity, magnetism, electric fields and so on which operate consistently. Sometimes they are called laws of nature, because careful observers over the years have not found any exceptions to these forces.
For example, whenever you drop something, it falls. Or whenever you combine two chemicals of a certain type, they always react the same way.
Human beings, however, tend to be unpredictable. This could be because they were granted free will by God (compare Determinism), or because predicting the behavior of complex systems is computationally hard.
The field of physical science concerns itself with discovering natural causes and codifying them in terms of the effects they have on the observable world. In astronomy, for example, Kepler introduced three laws of motion in the 1600s which still stand true today.
Issac Newton introduced his own, better-known, laws of motion as well. Newton told us that the amount of force applied to an object is directly proportional to the effect on its momentum. For example, if you push a 2,200 pound car on a railroad track with 110 pounds of force, it will accelerate at the rate of 1/20 meter/second. After 30 seconds, it will reach approximately 5 feet/second.
- Scott Aaronson, Quantum Computing since Democritus lecture notes. http://www.scottaaronson.com/democritus/lec18.html Retrieved 1/1/2012