Extra Words Ken-po Goku-i

The first line of the gokui is "A person's heart is the same as Heaven and Earth." The kanji that goes along with it is depicted in the graphic you see to the left side of this post.

My question is, "Where did the other English translation words come from?" What I am saying is this, "is this really the translation of the characters and if it is how did they determine that person and heart would be the same, and how is it the same, as heaven and earth. There is nothing notable to indicate that the person who wrote these characters originally meant it to be some comparison.

That comparison even if correct is not apparent, is it? This makes me wonder what exactly the Ancients intended when they provided the ken-po goku-i. I suspect, as stories are told, that because a good part of it was conveyed verbally and taking in to consideration the level of education for those who wrote things like the bubishi and lets not forget the environment, customs, and beliefs of those in that particular time will have an effect as well.

Maybe this was the ultimate purpose so that it would remain fluid so as times and people changed the meaning would change accordingly. The only hiccup here for me is the need to know the intent and meaning of those at their time so that I can utilize it to convey meaning to my time and meaning.