Psalm 19:7-8
The law of the Lord is perfect,
it revives the soul.
The rule of the Lord is to be trusted,
it gives wisdom to the simple.
The precepts of the Lord are right,
they gladden the heart.
The command of the Lord is clear,
it gives light to the eyes.
What then is this command of the Lord?
Deut 6:4-5
Hear, O Israel: The LORD our God is one LORD; and you shall love the LORD your God with all your heart, and with all your soul, and with all your might.
And what is the natural result of this?
Matthew 22:37
37] And he said to him, "You shall love the Lord your God with all your heart, and with all your soul, and with all your mind.
[38] This is the great and first commandment.
[39] And a second is like it, You shall love your neighbor as yourself.
[40] On these two commandments depend all the law and the prophets."
The command of the Lord is clear,
it gives light to the eyes.
or in the RSV
The commandment of the Lord is pure,
enlightening the eyes.
That pretty much sums it up. The rest is just commentary. :)