The mind of man is man's thinking organ. Through the mind, we know, think, imagine, recollect, and understand. The intellectual power, rationality, wisdom, and intelligence of man all belong to the mind. Broadly speaking, our mind is everything related to our brain. Mind is the psychological term, while brain is the physiological term. The mind in psychology is the brain in physiology. The mind occupies a big part in man's life because the mind directs man's conduct the most.
According to the Bible, man's mind is a battlefield. This is something unique. In the mind Satan and the evil spirits battle with the truth and also battle with the believer. We may use an example. The will and the spirit are like a fortress which evil spirits try to attack and capture. Man's mind is the field on which the battle takes place and from which the fortress is stormed and taken. The apostle said, "For though we walk in flesh, we do not war according to flesh; for the weapons of our warfare are not fleshly but powerful before God for the overthrowing of strongholds, as we overthrow reasonings and every high thing rising up against the knowledge of God, and take captive every thought unto the obedience of Christ" (2 Cor. 10:3-5). The apostle first told us about a war, then he told us where this war is and what it is for. This war is completely related to the mind of man. The apostle likened the "reasonings" to the "enemy's stronghold." He considered the mind as a stronghold guarded by the enemy that must be "overthrown." He also considered that within this stronghold there are many "rebellious thoughts." The apostle had to overthrow the mind of man and imprison and "take captive" the "rebellious thoughts" in man's mind, so these thoughts would be brought completely unto the "obedience of Christ." From these verses we can see that the mind of man is a battlefield because this is where evil spirits war with God.
The Bible tells us that "the god of this age has blinded the thoughts of the unbelievers that the illumination of the gospel of the glory of Christ, who is the image of God, might not shine on them" (2 Cor. 4:4). This agrees with the previously cited Scripture because this is how Satan guards and blinds the mind of man. Perhaps a man considers himself to be very intelligent, able to use various arguments to oppose the gospel. Some think that others do not believe because they do not yet understand. Actually, the eyes of men's minds have been covered up by Satan. Since men's minds are guarded by Satan, their "thoughts were hardened" (3:14). They were "doing the desires of the flesh and of the thoughts, and were by nature children of wrath" (Eph. 2:3). They are "enemies in [the] mind" (Col. 1:21) because "the mind set on the flesh is enmity against God" (Rom. 8:7).