One of the criticisms of the view of the trichotomous nature of man is that sometimes soul and spirit are used interchangeably. Although there is a consensus that the connotation of man’s “spirit” relates to God (who is Spirit) and soul connotes the psychological aspect (as indicated by the English term “psychology”), there is resistance to believing that man’s spirit and soul are ontologically distinct. Note that the primary use of these terms as nouns in Scripture (pneuma and psuche is consistent with their familiar use as English adverbs and adjectives.
The dichotomist view is that the nouns spirit and soul are merely synonyms. There is a difference in emphasis and function of man’s immaterial side. One of the main reasons for this view (that became mainstream after Augustine) is that sometimes these terms seem to be used in a synonymous or interchangeable manner.
The Reason for overlapping usage of spirit and soul
The reason for occasional overlapping of these terms is that, conceptually, just as the immaterial aspect of man is “in” the body, so the human spirit is “in” the soul. This model can be illustrated by concentric circles.
So, if a reference that may be expected to use the term “spirit” may use “soul” instead, this would not contradict a more precise meaning of these terms.
A biblical illustration of the subtle distinction of spirit and soul is the Old Testament Tabernacle (and later, the Temple).
The Tabernacle was one building, yet with two distinct rooms. Only a curtain separated them. Similarly, the believer, who is a “temple of the Holy Spirit” (1 Cor. 6:19) has two aspects of his immaterial being–soul and spirit. The whole Tabernacle would be considered The Holy Place, yet the innermost room was distinct as The Holy of Holies.
A Defense of Trichotomy in light of some overlapping of terms
In another post (Sept. 4, 2022), Lehman Strauss made the case for man as spirit, soul and body by noting that he is made in God’s image. God is three-in-one, the Trinity. Now we consider a parallel between our biblical evidence for God being Triune and man as a triune being created in His image.
If some interchangeable use of soul and spirit disproves man as trichotomous, the same argument could be used to dispute the doctrine of the Trinity (and we accept the biblical view of God as triune). Consider these theological basics.
God is one: “Hear, O Israel: The Lord our God, the Lord is one!” (Deut. 6:4).
The first person of the Godhead is the Father (John 5:16-45). The Father and the Son are distinct ontologically, yet are one God (Heb 1:1-3; John 11:1,14).
The Father and the Son are ontologically distinct from the Holy Spirit. “And I will pray the Father, and He will give you another Helper, that He may abide with you forever— the Spirit of truth, whom the world cannot receive, because it neither sees Him nor knows Him; but you know Him, for He dwells with you and will be in you” (John 14:16,17).
The Father, Son, and the Holy Spirit are distinct persons in the Godhead. “When He had been baptized, Jesus came up immediately from the water; and behold, the heavens were opened to Him, and He saw the Spirit of God descending like a dove and alighting upon Him. And suddenly a voice came from heaven, saying, “This is My beloved Son, in whom I am well pleased” (Matt. 3:16; see Acts 1:4,5).
Now we observe that the names of the Godhead are sometimes used interchangeably.
The Son is prophetically called “The Everlasting Father” in Isaiah 9:6. In John 10:30 “Jesus declared ‘I and My Father are one.'” John 14:8,9 records, “Philip said to Him, ‘Lord, show us the Father, and it is sufficient for us.’ Jesus said to him, ‘Have I been with you so long, and yet you have not known Me, Philip? He who has seen Me has seen the Father; so how can you say, “Show us the Father”‘?”
God the Father is “spirit” and “holy.” “God is Spirit, and those who worship Him must worship in spirit and truth” (John 4:24). “Holy Father…” (John 17:11).
The Holy Spirit is sometimes called the Spirit of Christ. “Of this salvation the prophets have inquired and searched carefully, who prophesied of the grace that would come to you, searching what, or what manner of time, the Spirit of Christ who was in them was indicating when He testified beforehand the sufferings of Christ and the glories that would follow” (1 Pet 1:10,11).
“But you are not in the flesh but in the Spirit, if indeed the Spirit of God dwells in you. Now if anyone does not have the Spirit of Christ, he is not His. And if Christ is in you…” (Rom. 8:9,10)
“And because you are sons, God has sent forth the Spirit of His Son into your hearts, crying out, ‘Abba, Father!’” (Gal 4:6. See Gal. 2:20).
The Son is described as life-giving spirit. “And so it is written, ‘The first man Adam became a living being.’ The last Adam [Jesus, Son of God] became a life-giving spirit” (1 Cor. 15:45).
Although the names of the Godhead are sometimes used interchangeably, God’s unity is maintained: “Go therefore and make disciples of all the nations, baptizing them in the name [singular] of the Father and of the Son and of the Holy Spirit” (Matt. 28:19).
Since the occasional interchangeable use of the terms Father, Son (Christ) and Holy Spirit does not disprove God’s triune nature (the Trinity doctrine), even so, some interchangeable use of soul and spirit does not disprove man as triune (ontologically spirit, soul and body). 
 For more on the symbolism of the Tabernacle and sample quotations from five authors, see this blog site’s heading–“The Tabernacle.”
 The three aspects of man need not require a de-emphasis of his fundamental unity. We maintain that man is one in personhood, with two separable sides, yet three distinguishable aspects — holistic trichotomy. Just as the dichotomist can emphasize the holistic nature of man while accepting the biblical teaching that he has a material side and immaterial side (e.g., 2 Cor. 5:8), so the trichotomist can and should be holistic as well.
 This conclusion is based on inductive biblical theology, the testimony of expositors, and is consistent with the view of the early church (before Augustine).