[newt]

"Simple" is a subjective term.

Which is why it's sometimes stated as "Entities should not be multiplied unnecessarily" - i.e pick the explanation with the fewest moving parts. That's less subjective, assuming you can agree on how to count the entities.

1. Things always fall down

This is not actually an explanation at all, it just restates the observation.

2. There exists gravitational force between bodies

Newton's inverse square law of gravitation is simpler than relativity, and it's good enough for a lot of uses. But it is not a viable explanation either, since it does not fit exactly with observed reality - it does not handle the edge cases of very fast or very heavy stuff.

[hasenj]

> This is not actually an explanation at all, it just restates the observation

It's an explanation for "why the pen fell down". It states a law whereby "things always fall down".

[newt]

To go further, What is and what isn't an "explanation" ?

"Things always fall down" doesn't add much. It says that the observation can be repeated.

Newton's law says more than "There exists gravitational force between bodies" which isn't much of an improvement over "things always fall down".

Newton provided a mathematical formula to measure it, which, as best as could be seen in Newton's time, fit exactly with reality, applying from apples up to planets. http://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gra...

This makes testable, measurable predictions about "why things fall down", i.e. "things fall down because all things obey the law of universal gravitation as given in the formula...". But it doesn't even begin to explain why there is gravity all rather than no gravity. Science is largely silent about this category of question of meaning not measurement - it observes reality and predicts based on extrapolating from existing observations. Other explanations of "why" are hardly much better - "gravity exists because God said so" aren't very satisfying since you can't test it or infer anything from it.

[newt]

Ok, it generalises the observation to other things as well.