Question:James 3 seems to be talking about keeping our mouth under control and not to boast about everything we can do. What is it that comes from your mouth and corrupts the whole body? To speak good things and bad things is wrong. It has to be one way. It also compares the earthly wisdom (which I guess would be envy and strive which is bad) against the wisdom from above (which is pure).
James 3 discusses the need for Christians to control the words which come from their mouths. But those words are merely the reflection of a person's thoughts and attitudes.
Verse 1: It is especially important for those who are teaching others. They incur a greater responsibility. They are not only accountable for what they say but are also responsible for how people behave because of what they taught. An example of this would be the prophet Balaam, who told the king of Moab how to lead Israel into sin so that God would destroy Israel. The people who sinned were responsible for their own sins, but Balaam was also held responsible because he inspired the sins. See the sermon outline "The Story of Balaam" for more details.
Verse 2: While we need to control our tongue, the practical view is that we will all fail at times, just like every falls prey to sin. "If we say that we have no sin, we deceive ourselves, and the truth is not in us" (I John 1:8). This doesn't mean we don't try. It means we must appreciate the magnitude of the problem before us.
Verses 3-6: James proves that size doesn't indicate the power something has. The tongue reflects the thoughts in our mind. Thus the way we talk often reflects the way we go. "For from within, out of the hearts of men, proceed evil thoughts, adulteries, sexual sins, murders, thefts, covetings, wickedness, deceit, lustful desires, an evil eye, blasphemy, pride, and foolishness. All these evil things come from within, and defile the man" (Mark 7:21-23). Sins don't just happen. They are contemplated, they are talked about, long before they are actually done.
Verses 7-8: We cannot let our guard down. The tongue (and the thoughts it reflects) can never be fully controlled. "Therefore let him who thinks he stands be careful that he doesn't fall" (I Corinthians 10:12).
Verses 9-12: Man is inconsistent. It is something that ought not to be, but it is.
Verses 13-18: People tend to behave in the manner in which they think. Problems in a person's life, the way he behaves and the sins he commits, are a reflection of the person's attitude. Jesus uses the same point to tell us how to spot a false teacher. "Beware of false prophets, who come to you in sheep's clothing, but inwardly are ravening wolves. By their fruits you will know them. Do you gather grapes from thorns, or figs from thistles? Even so, every good tree produces good fruit; but the corrupt tree produces evil fruit. A good tree can't produce evil fruit, neither can a corrupt tree produce good fruit. Every tree that doesn't grow good fruit is cut down, and thrown into the fire. Therefore, by their fruits you will know them" (Matthew 7:15-20).
When you see discord, arguments, and strife between people, you can be sure that one or more of the people involved have an improper attitude toward other people.