Our Scripture reading today is Luke 24:38-39.
Verse 38, "And he said unto them, Why are ye troubled? and why do thoughts arise in your hearts?"
The Lord Jesus was able to see into the minds of those who were around Him.  They were not so comfortable being around the risen Lord.  Was is it really Him?  It must be just a spirit.  All kinds of fearful thoughts were going on in their minds.  
It is very important that we guard what thoughts are allowed to remain in our minds.  There will be times of casting down certain thoughts that are overwhelming us.  2 Corinthians 10:5, "Casting down imaginations, and every high thing that exalteth itself against the knowledge of God, and bringing into captivity every thought to the obedience of Christ;."  
The Apostle Paul gives a specific list of things we should be thinking about in Philippians 4:8, "Finally, brethren, 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; if there be any virtue, and if there be any praise, think on these things."  We would save ourselves much fear and worry if we applied this verse more in our lives.  By God's grace, we can have healthy thinking.