Saint Hesychios sees watchfulness as follows:
Watchfulness is a continual fixing and halting of thought at the entrance to the heart... If we are conscientious in this, we can gain much experience and knowledge of spiritual warfare.
He shows us that this involves an effort to intercede on our thoughts, forcing them to be examined, to shine the commandments of our Lord on them. He sees this as warfare, which emphasizes the nature of this effort. We know in warfare we need to have effective weapons that are stronger than those of the enemy.
Another church elder from modern times, Piasios, tells us about some of the consequences of not being watchful.
“When our soul lives carelessly without watching over its thoughts, it will consequently fill up with dirty and sly thoughts.
As a result, people start developing psychological problems which gradually pile up.... Some people, while they are found in this situation and come face to face with the problem itself, they do not realize it, and thus are unable to humbly confess to their spiritual father their fall. Instead, they look for a “secular” solution and consult a psychiatrist, who will inevitably prescribe medication... The only solution is to become aware of the problem and confess it to a spiritual father and then humbly follow his advice.
In our days, people have lost control over their lives, and they do not know what they are doing. The reason being, that they do not wish to be guided; they want to live undisturbed, following their own free will, which will eventually bring their total destruction... when man uses his freedom and independence without taking into consideration his human weakness, he becomes deceived; he experiences and interprets everything by using his own logic. Instead of God’s grace, human logic rules his life, and his mind is in confusion. This is terrible.”
It is essential to build self-control. Most importantly, we have to learn to harness the actions of our mind which tends to run wild and unchecked. This condition forces us to rely on mental programming that needs to be changed to live the Christian life. This is what being watchful is about. Being watchful means that we have the necessary self-discipline where we are able to guard our inner sanctuary from being invaded by thoughts stimulated by our senses that lead us to sinful actions. It is an ability to intervene in the process of choosing how to act based on any kind of stimulus that leads to a thought.