To the law and to the testimony! If they do not speak according to this word, it is because they have no dawn. (Isaiah 8:20, NASB)
Who do you listen to? Where do you get your wisdom and advice? And how do you know it's a trustworthy source? Isaiah helps us with that. He tells us where to go for direction and who to ignore.
Where to go. We must go to Scripture. That's what Isaiah means by “the law” and “the testimony.” Scripture shows us who God is and what He has done (the testimony). It also shows us what He requires (the law). Therefore, to know Scripture is to know God and His ways.
Who to ignore. We are shown the futility of listening to those who speak in ignorance or negligence of God's Word. Isaiah says they have no dawn. Without the revelation of Scripture there is no light. The light reveals. The darkness conceals. These voices speak from the darkness, where they desperately poke around hoping to stumble upon some wisdom. They are blind guides.
So, where do you get your wisdom? Where does your life advice come from? Be wary of listening to anyone who does not speak from the Book.