A sound mind has the mind and attitude of Jesus. The human mind was never created to be used outside of a submissive relationship with God, but rather to be in submission to the Holy Spirit.

2 Timothy 1:7 For God hath not given us the spirit of fear; but of power, and of love, and of a sound mind.

The human mind was never created to function without the flowing of the Holy Spirit to guide, direct, and empower it.  Without Christ, there can be no wisdom, understanding, or righteousness.  When anyone is walking in foolishness or wickedness, they are not in the Spirit of Christ.  They are functioning in the realm of the natural no matter how spiritual that they claim to be.