Notes from Proverbs 4:23-27
For from it flow the springs of life.
24 Put away from you a deceitful mouth
And put devious speech far from you.
25 Let your eyes look directly ahead
And let your gaze be fixed straight in front of you.
26 Watch the path of your feet
And all your ways will be established.
27 Do not turn to the right nor to the left;
Turn your foot from evil. (NASB)
Notice the similarity of the above passage with Romans 3:10-18. The description in that passage is of a people who have turned away from God, and the members of their bodies (with all the theological significance these bodily members hold) have become corrupted.
Pastors/theologians have equated this term with "mind" or "the seat of emotions." Jonathan Edwards defined the heart as the seat of desire. In his view (which I think is correct), we choose according to our greatest desire and our "heart" is what determines our desires. It is for this reason that the Holy Spirit must renew our heart before we will repent of sin and believe in Jesus.
Whatever the precise definition, it is clear that the "heart" is what is at the core of our being.
The believer must carefully guard his or her heart making sure that our awareness, attitudes, appetites, affections, and aspirations are being conformed to the Word of God.
What comes out of the mouth is seen as a direct reflection of what is in the heart.
18 "But the things that proceed out of the mouth come from the heart, and those defile the man. 19 "For out of the heart come evil thoughts, murders, adulteries, fornications, thefts, false witness, slanders. (Matt 15:18-19 NASB)
And in writing of the sinlessness of Jesus, Peter emphasizes that "no deceit was found in His mouth" (1 Pet 2:22).
I believe that in these verses, the "eyes" indicate our focus: in the sense of what we dwell upon or the goals that we strive after. So Hebrews 12:2 speaks of fixing our eyes on Jesus, the author and perfecter of our faith.
As Dr. Orrick described when he preached from Galatians 5 on walking by the Spirit, the "feet" are indicative of a persons life. (So that the biblical expression of "walking with God" indicates living for God and in communion with Him.)