Narkissos.....I've been thinking more about the "scripture" in James 4:5 and why it is cited in this context. First of all, regarding the Society's interpretation of the verse given in Dunlap's Commentary on the Letter of James and in the 1977 Watchtower, where they state that the "spirit" is merely man's spirit or inclination for jealousy, I don't think this is consistent with the punctual aspect indicated by the aorist verb katókisen "dwelt down," which instead would suggest a "spirit" (e.g. the Holy Spirit) that came to dwell at a specific point in time, and not the spirit that has been part of us all along. Moreover, the strong parallels between this passage and Hermas (cf. Mand. 3.1, 10.2.4-5) and Numbers 11:29 are also important because both refer to the indwelling of the "Holy Spirit" or the "Spirit" of prophecy in the flesh.
But why is the obscure scripture in v. 5 cited at this point in the discourse? I think the preceding verse gives us some solid clues:
"Adulterers (moikhalides), don't you know that friendship with the world is hatred toward God? Anyone who chooses to be a friend of the world becomes an enemy of God. Do you suppose it is in vain that the Scripture says, 'He yearns jealously (pros phthonon) after the spirit (pneuma) which he has made to dwell (katókisen) in us?' But he gives all the more grace; therefore it says, 'God opposes the proud, but gives grace to the humble.' Submit yourselves therefore to God. Resist the Devil and he will flee from you" (James 4:4-7).
There is a strong dualism here between God and the Devil and between being a friend of the world and being an enemy of God. Such dualism is explicit in apocalyptic writings like 1 Enoch (which has a "Two Ways" section) and in the eschatological portions of Matthew, which involve an end-times dualistic division of the world (cf. 25:31-33). Deuteronomy 30:15-20 has Moses giving the "Two Ways," one being a curse and the other a blessing. In the logic of the "Gog and Magog" oracles attributed to Eldad and Modad in the Targums, those who side with "Magog" (= "the world") would indeed be "enemies of God" and would be destroyed by the Lord at his "coming". The reference to people who forsake God as "adulterers" is a common one in the OT prophets (cf. Isaiah 57:3; Jeremiah 3:6-10, 23:10; Hosea 7:4), and I am struck by the possible connection with Numbers:
"If a man's wife goes astray and is unfaithful to him by sleeping with another man, and this is hidden from her husband and her impurity is undetected (since there is no witness against her and she has not been caught in the act), and if a spirit of jealousy (pneuma zélóseós) come over her husband and he suspects his wife and she is impure, or if he is jealous (zélósé) and suspects her even though she is not impure, then he is to take his wife to the priest....This, then, is the law of jealousy when a woman goes astray and defiles herself while married to her husband, or when spirit of jealousy come over him (ep' auton) because he suspects his wife" (Numbers 5:12-14, 29-30).
The close proximity between ch. 5 and ch. 11 of Numbers and the similar linkage between "spirit" and "jealousy" makes me wonder whether, in a midrash interpretation which would have noticed the coincidence and exegetically linked the two proximal passages together, those who were jealous of Eldad and Modad's prophetic gifts would have had the "Spirit of Jealousy". Or is it that God, who gives out his Spirit, is jealous of those who "go astray" like adulterers and is himself the one who yearns "jealously"? This would comport well with Exodus 34:14, which specifically deals with those who leave God for idolatry. On the other hand, maybe the connection is friction between the "spirit of jealousy" and the "spirit of prophecy," since Hermas has this to say: "An angry temper is first of all foolish, fickle, and senseless. Then from foolishness comes bitterness, and from bitterness wrath, and from wrath anger, and from anger vengefulness; then vengefulness, being composed of all these evil elements, becomes a great and incurable sin. For when all these spirits dwell in one vessel, where the Holy Spirit also dwells, the vessel cannot contain them, but overflows. So the sensitive Spirit, which is not used to living with an evil spirit nor with harshness, departs from a person with the evil spirits" (Mand. 5.4-6).
I think this latter concept works very well with the concept in James. Those who forsake the faith are like adulterers and when the Spirit departs from them, they yearn jealously for the Spirit and are jealous at those who still have the Spirit. The concept in the Eldad and Modad story in Numbers is also of Joshua being jealous at Eldad and Modad having the Spirit. There are differences tho -- one being jealous in ch. 5 is the husband, not the adulterer. So the relation may be more complex. But in any event, this additional observation strengthens my suspicion that a midrash or pseudepigraphon dependent on Numbers is involved here.