The key to the whole man is his mind. According to an annotation in Thomas Nelson's King James Study Bible, the word HEART was often translated as MIND, because the Hebrews did not have a specific word for our intellect.
Whatever captures the mind, captures the man.
For me, knowing this really adds depth to certain related passages in the Bible. For example:
Oh generation of vipers, how can ye, being evil, speak good things? For out of the abundance of the heart the mouth speaks. A good man out of the good treasure of the heart brings forth good things: and an evil man out of the evil treasure brings forth evil things. But I say unto you, that every idle word that men shall speak, they shall give account thereof in the day of judgment. For by thy words thou shalt be justified, and by thy words thou shalt be condemned.
Matthew 12:34-37 (KJV)
Luke records Jesus' statement from Matthew 12 this way.
A good man out of the good treasure of his heart brings forth that which is good; and an evil man out of the evil treasure of his heart brings forth that which is evil: for [out] of the abundance of the heart, his mouth speaks.
Luke 6:45 (KJV)
Therefore, if the heart is equivalent to the mind, then it follows that we will be held equally accountable for our thoughts as well.
So far, we have come to understand that what we say and what we think is rooted in our heart. Hence the admonition to guard our heart makes sense. But what makes our heart good or bad? Is it limited to our thoughts and how we express them? Of course not. Just prior to the passage I quoted from Matthew, Jesus also had this to say.
Either make the tree good, and his fruit good; or else make the tree corrupt, and his fruit corrupt: for the tree is known by his fruit.
Matthew 12:33 (KJV)
Now we have even more reason to strongly consider why we need to 'guard our heart with all diligence'. For Christ makes it clear that we 'bear fruit' according to the type of tree we have chosen to become. How is this possible, you ask?
Well, contrary to the Reformed, Calvinist teaching that prior to Creation, God pre-determined who will be saved and who will not, Christ makes it clear that we choose our own destiny. We choose whether we will be 'good' or 'corrupt'.
Rather than owning up to the poor choices they have made in their lifetime, many people will lay the blame on various circumstances, along with cultural and societal influences. Especially, when they find themselves failing to measure up to the life they know they should be living, but rejected. The life that God created for us and calls us to live out, for our benefit and His glory. But in all truth, the choices we have made, and will make in the future, come from our desire to fill a perceived need within us. More often than not, we choose the easy route to instant gratification.
What makes it worse, is that we instinctively know when we are making a poor choice. But we do it anyway, because we have believe we do not deserve nor will ever be able to attain and maintain the lifestyle God desires for us. The life that He has called us to.
We use the excuse the we are only human, instead of believing that we are Children of the Most High God who created us in His image. We allow the false and empty promises of this world to convince us that "A bird in the hand is worth two in the bush." So rather than risking being shunned, ostracized, and rejected as some sort of religious fool, we become a fool believing that it is better to seek the treasures of the world, instead of the greatest treasure of all. That treasure is eternal life in the Kingdom that God has prepared for those who overcome.