Natural causes
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.
In the biological world, there are natural laws governing plant growth, animal behavior, infectious diseases and so on.
Human beings, however, tend to be unpredictable because they were granted free will by God (compare Determinism).
Physical science
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.
