One of the ways that we know that we are being discreet in life is by what comes out of our mouths when we speak. What is fascinating about this passage is that the overall gist of the fifth chapter of Proverbs is that it deals primarily with the issue of immorality and those who fall into the trap the adulteress.
One of the reasons why we need to gain wisdom and understanding is so that we may watch out to be discreet. The word "observe" in this passage means to guard and be careful. Zhodiates states that this word basically means to be on your guard. What you are guarding is "discretion." Yet the word is different than the discretion that is mentioned most often in the book of Proverbs. It means a thought - or the thoughts in our minds that are used to make up our plans. There is one major point though that needs to be distinguished about this word. It most often means evil plans and schemes that are contrary to God's ways. What Solomon is warning his son to watch out for in life is evil plans and schemes that begin to formulate in his mind.
When it comes to maintaining purity in our lives, what happens in our heads is vitally important. We need to watch over our minds and what is running through them. Actions begin as thoughts - which in time turn into plans and schemes. The reason we watch and guard our minds is to make sure that when lust and immoral thoughts try to push in - we deal with them. It is not just enough to push these thoughts out - we need to bring them to the cross. The Word tells us to mortify these things - and that means to kill them! Nothing short of this will do.
The second half of this proverb says that when we watch over the development of evil plans and schemes in our heads - it will help us to watch over our mouths. Our mouths will speak "a knowing" when we guard our minds. This knowing is once again primarily a discernment - but one that is specific to the situation. Remember that here this refers to sexual immorality and the wiles of the adulteress. We are warned in the very next verse that the adulteress captures men with her words and her flattery. These things often cause men to be deceived in their hearts and minds. The flattery of a woman giving them attention strokes their pride and their ego - something very dangerous to a man who doesn't watch what enters and walks through his mind. In time his pride and ego can lie to him and fill his mind with thoughts of further flattery. He begins wanting to spend more time with this woman - at lunch and eventually at secret meetings he keeps from his wife. We all know where this is going - toward an eventual adulterous relationship with this woman. Yet the real danger started with the thoughts that ran through his mind - and the words he began to speak to this woman because he didn't watch and guard against the adulterous thoughts that were fostered in his head. He may not wake up until the damage is irreparable.
It is vital to our spiritual progress and protection to keep a close eye on what is going through our minds. This is the stuff that can cause us serious problems. What begins in just a seed form in our minds will eventually sprout and bear disastrous fruit. Better to deal with seeds by being wise - than have to cut down entire forests because of a lack of watchfulness.