The principle means that something is determined - like the formula in arithmetic whereby 1 + 1 = 2; as long as the question (and definitions of factors) remains the same then the result will always be exactly the same.
I can state that the fixed principle of the world is that things make results repeatedly through what is inside themselves.
We can find that everything that exists in the world does so through actions, and the structure which we can verify is produced through the process of activities is that of an object which moves through three states (dimensions), changing itself depending on the specific things (causes) in the object.
A clear and simple example is that of fruiting trees. An apple tree only knows how to make apples because that's what's encoded inside itself. Through the process of its life, if it can produce a fruit with seeds, thereby keeping its own origin, it can be reborn again if it's got the right environment.
The golden rule is not so helpful because the problems to necessitate the rule are not present in the rule itself (like commandments). People who don't already have virtue can't awaken their conscience through the golden rule (to get the result - morality) because the very source of the solutions (the problems) are missing. Instead, when people learn and correctly understand the principle they don't need to be told to do good and not to do bad, they naturally start to recognize from what causes good and bad results happen to themselves, so they don't want to do anything that would harm themselves after they confirm.
Then how did Buddha himself learn it?
With that foundation, I can answer your question a little more directly by telling you that he was able to realize Buddha's teaching through the world itself, after he opened his own eyes. (Buddha's teaching means how the world is operated, so everything is its evidence and can be found through any examples)
If you have any doubts please keep asking questions :) This can be dangerous territory if mistreated
Also if an Enlightened being sees the world as it is and gets perfect understanding directly, he(she) can make what is generally considered scientific breakthroughs with ease. Does it really happen?
Yet, despite his extensive travels, it seems so easy for people who claim philosophical or scientific knowledge to criticize him before they verify who he is and what he really can perceive.
Psychologically speaking, why do people behave like that? In my experience it occurs when they don't want their falsehoods to be revealed.
Now here's where it gets interesting. The mind is fully contained in the universe - it is physical. So emotions, thoughts and feelings are all things. They are emergent behaviors of this physical reality. Hence the dual mode of thought being incorrect - there is no distinction between objective and subjective, they are one and the same. So if the universe is deterministic and our experience is physical, then there is no free will.
