"He that is of a cool spirit is a man of understanding" (Prov. 17:27, ASV).
Our spirit needs to be fervent. It also needs to be cool or calm. Fervency is related to not being "slothful in zeal...serving the Lord"; coolness is related to knowledge.
If our spirit is not cool, our actions will often be uncontrolled. The purpose of the enemy is to cause the saints to go astray and lose contact with the Holy Spirit. We often see that when a saint’s spirit is not cool, he changes his living from one which is according to principle to one which is according to emotion. The spirit and mind originally were closely linked together. As soon as the spirit is not calm, the mind becomes provoked; as soon as the mind becomes fervent, the believer loses control of his many actions which then become abnormal. Therefore, it is always profitable to maintain a calm spirit. In order to keep our walk always on the path of the Lord, we have to constantly ignore excitement in the emotion, an increase in our desire, and confusion in the mind; instead, we should retreat to ponder every question in our calm spirit. If we act whenever our spirit is provoked, we fear that all the resulting actions will be against the will of God.
Because of our knowledge of the self, God, Satan, and our thorough perception of all things, we should have a calmness in the spirit which soulish believers never have. The Holy Spirit should fill the believer’s spirit. The soul should be completely put to death so that the spirit can have an unspeakable calmness. Regardless of any changes in soul, body, or environment, the calmness within the spirit will definitely not be lost. Just like the sea, regardless of how much the waves roar on the surface of the sea, the bottom of the sea is always very quiet and still. Before a believer separates his soul from his spirit, whenever something happens unexpectedly, his whole person immediately becomes confused, bewildered, or at a loss as to what to do or is at least shaken in his purpose. This is because of the lack of spiritual knowledge and the lack of separation between the soul and spirit. Therefore, in order to maintain a separation between the soul and the spirit, he must maintain a coolness in the spirit. The believer will then have an "unshaken" experience. No matter how disturbing the outward circumstances are, they cannot cause him to lose the calmness and peace within. Even if a mountain collapsed in front of him, he would not lose his calmness. This is not gained by man’s meditation but by the believer’s reliance on the Holy Spirit’s revelation of the real condition of all things and by the restriction of the believer’s soul. This prevents the soul from controlling his spirit.
The matter we are discussing relates to the control of the will. Our spirit should be subject to the control of our will. Our will desires fervency; it also desires calmness. We should not allow our spiritual condition to go beyond what our will is able to control. It should be fervent in the Lord’s work, but it should also maintain a cool attitude when doing the Lord’s work.
(Spiritual Man, The (3 volume set), Chapter 25, by Watchman Nee)