Trichotomists believe that man is composed of three parts: the material body, the immaterial soul and the immaterial spirit. Dichotomists believe that man is composed of two parts: the material body and the immaterial soul/spirit. What does the biblical evidence say?
"For the word of God is living and active, sharper than any two-edged sword, piercing to the division of soul (psychē) and of spirit (pneuma), of joints (armos) and of marrow (muelos), and discerning the thoughts (ennoia) and intentions of the heart (kardia)." (ESV, NA28*)
The anthropological key concepts that are used in this text are pneuma (πνεῦμα), psychē (ψυχὴ), armos (ἁρμός) meaning "joint," muelos (μυελός) meaning "marrow," enthumēsis (ἐνθύμησις) meaning "thought," ennoia (ἔννοια) meaning "thinking," and kardia (καρδία).
1 Thessalonians 5:23
"Now may the God of peace himself sanctify you completely and may your whole spirit (pneuma) and soul (psychē) and body (sōma) be kept blameless at the coming of our Lord Jesus Christ." (ESV, NA28**)
If we take pneuma, psychē, sōma in the Greek to provide for the three parts, then what shall we do with the other anthropological terms? And why choosing these three anthropological terms as basis for the nomenclature and not more terms, for example nous (νοῦς), pneuma, psychē, sōma or sarx (σάρξ), which occur in other passages? The dichotomists then classify pneuma and psychē as synonyms, but this does not solve the problem because if I have two parts why not calling them nous and sōma, or sarx and kardia? Why not suggesting as many parts as there are anthropological terms?
Hebrews 4:12 has more than three terms, so why dividing into two or three parts? Sōma is not even used in Hebrews 4:12, so based on what is sōma then one of the three? There are 7 anthropological concepts in Hebrews 4:12, so why not seven parts of the human make-up?
The message of Hebrews 4:12 and 1 Thessalonians 5:23
The point of both texts is, actually, the opposite of what they are used for when trying to demonstrate a human make-up. In both texts the point is unity. In Hebrews 4:12 we need to be entirely pure for the Lord. In 1 Thessalonians 5:23 the wish is that we are doing entirely well. This is the idea of šālôm (שָׁלוֹם). Examples are given of aspects of the human existence in these two texts, other examples could be given to say something about the human existence. We need to live in purity, as defined by God’s word, examining our way of living. God desires our well-being in all aspects of our life.
* Hebrews 4:12 (NA28): Ζῶν γὰρ ὁ λόγος τοῦ θεοῦ καὶ ἐνεργὴς καὶ τομώτερος ὑπὲρ πᾶσαν μάχαιραν δίστομον καὶ διϊκνούμενος ἄχρι μερισμοῦ ψυχῆς καὶ πνεύματος, ἁρμῶν τε καὶ μυελῶν, καὶ κριτικὸς ἐνθυμήσεων καὶ ἐννοιῶν καρδίας·"
**1 Thessalonians 1:23 (NA28): "Αὐτὸς δὲ ὁ θεὸς τῆς εἰρήνης ἁγιάσαι ὑμᾶς ὁλοτελεῖς, καὶ ὁλόκληρον ὑμῶν τὸ πνεῦμα καὶ ἡ ψυχὴ καὶ τὸ σῶμα ἀμέμπτως ἐν τῇ παρουσίᾳ τοῦ κυρίου ἡμῶν Ἰησοῦ Χριστοῦ τηρηθείη."