The heart - As indicated in Appendix 1 this represents the inward part of Man in contrast to the outward (Romans 2:28/29). It has a wider meaning than "the affections", which is the meaning that is often attached to it today. It is, being inside Man, his hidden part and so it is used in Scripture. For examples see: "The hidden man of the heart" (1 Peter 3:4) and "Thy
word have I hid in my heart" (Psalm 119:11). (See also Psalm 40:10) Being inside Man it can be said to be the core or centre of his being, the fount in man of his thoughts (Matthew 15:19) and feelings, so we have it said:" My heart is welling forth [with] a good matter" (Psalm 45:1) It is sometimes almost synonymous with mind (e.g. Luke 2:19) though that is rather limited to Man's intellect as distinct from his emotions (feelings of fear, longings etc.).
The bowels - These are Man's lower parts in contrast to his higher parts and are used for strong feelings, whether of pain or otherwise (Job 30:27; Psalm 22:14; Song of Songs 5:4; Jeremiah 4:19).
The mind - This is Man's intellect (his thinking part) and stands in contrast to his other parts (the unthinking parts). Persons who are mad are said to be out of their mind, beside themselves (Mark 3:21) or off their head, rather than out of their heart, soul or spirit. The contrast is to have a sound mind, that is, be sensible (Mark 5:15; Luke 8:35). The mind is necessary in
order that we may apprehend or perceive things (Romans 1:20).
The head - This stands in contrast to the feet, that is, the top to the bottom (Isaiah 1:6; 1 Corinthians 12:21). Sometimes the contrast is to the tail (Deuteronomy 28:13; Isaiah 9:14/15). The thought in both cases is similar - the extremities are indicated. The head contains Man's physical brain (his thinking faculty). Thus we have: "The visions of my head troubled me" (Daniel 7:15). (See also Daniel 4:5)
The spirit - This is in contrast to the flesh (see Appendix 1). The immaterial in contrast to the material. It is what I am in when I am out of the body (1 Corinthians 5:3; Colossians 2:5; Revelation 17:3). One cannot in the same way speak of being "in soul", because the soul is the ego (the self).