Our voluntary thoughts not only reveal what we are , they predict what we will become . Except for that conduct which springs from our basic natural instincts, all conscious behavior is preceded by and arises out of our thoughts. Thinking stirs feeling and feeling triggers action. What was penned there is true in the negative as well as the positive. Would anyone doubt the poisonous effects that harbored thoughts feeding unforgiveness, anger, hatred, revenge, envy, jealousy, greed, dishonesty, lust, etc.
Thoughts have great potential for good or evil, but only when pondered.
The Greek shows that it was the pondered thoughts that Jesus referred to. Also, please notice the inference that it is our choice to entertain them or not to entertain them. Lot's wife made the serious and fatal mistake of choosing to ponder thoughts in her heart of her past life in Sodom , even after her deliverance. Our pondered thoughts will build us up or tear us down spiritually . This is explicitly shown with the Prodigal Son.