If you don't use the mind that God has given you, the devil will. He loves an idle mind and will try to reek havoc in your thought life if you don't take command of your thoughts. I'm not advocating that we have to be busy our every waking hour, but we must guard against being idle minded.
If you choose to sit around doing nothing, the devil will try to contaminate your thoughts with his lies. He might start with subtle things but believing even one small lie will begin to contaminate your thoughts.
You must know the word of God to discern when that ole' serpent is trying to lie to you. And you must replace those lies with truth to purge them out of your life. Extreme Example: If somehow the devil was trying to pursuade you to kill someone, then your weapon would simply be, "Thou shall not kill."
When Jesus was tempted by the devil in the wilderness, his only reply was the word of God. The truth banishes the devil's schemes.
If you feel your mind has been contaminated with unhealthy thoughts, it can cause confusion and a ton of anxieties and even illness. But you can renew your mind by the reading of God's word. God's word will purge your thoughts and cleans your heart. There is power in the word of God to bless you with wisdom and knowledge and the ability to discern when thoughts are contaminating and coming straight from "the author of lies".
Fill your mind with productive things. The Lord advises us, "And finally bretheren, whatsoever things are true; whatsoever things are honest; whatsoever things are just; whatsoever things are pure; whatsoever things are lovely; whatsoever things are of good report; be there any virtue; be there any praise, think on these things."
Don't be a pawn for the devil's wicked devices. Take control of your thoughts by being productive. Don't waste valuable time. We must be good stewards of time as well as money. Read a book, take a walk, say a pray, talk to God.
There are many productive things you can do when you set your mind to it. Obviously I'm not talking to everyone, but if the shoe fits, wear it. Because if you don't use your mind, you may just lose it.