The verse in question resembles what many scholars have termed pre-literary creeds and hymns (See, e.g. How Jesus Became God: The Exaltation of A Jewish Preacher from Galilee by Bart Ehrman, HarperCollins, New York, NY, 2014, p. 216).
If this is the case, we can arrange 12b poetically, like so:
...soul and spirit...
...joints and marrow...
...thoughts and intents...
Or, more formally:
This shows that soul (A) corresponds with joints (A1) and thoughts (A2) while spirit (B) corresponds to marrow (B1) and intents (B2).
As such, it's a clear indication of a parallelism, common to Biblical poetry.
So then, how to define or understand "soul" and "spirit"? We can define the two terms in light of their poetic parallels.
A quote from James Kugel as found on p.19 in The Art of Biblical Poetry: Revised and Updated by Robert Alter (Basic Books, Philadelphia, 2011), can shed some light here:
"B [is] connected to A, had something in common with it, but was not expected to be (or regarded as) mere restatement...for it is the dual nature of B both to come after A and thus to add to it, often particularizing, defining, or expanding the meaning, and yet to harken [sic] back to A and in an obvious way connect to it."
What this means is that while spirit is no "mere restatement" of soul, since spirit comes after, it can particularize, define, or expand the meaning, of soul. The same is true, then, for joints and marrow, and for thoughts and intents.
Additionally, since 12b is a poetical tristich in parallel, all the words in both category A and category B help to either particularize, define, or expand the meaning of their cross-category correspondence.
With this in mind, an analysis:
From Kugel and Alter, we can say that:
- Spirit is a particularization, definition, or expansion of the word soul
- Marrow is a particularization, definition, or expansion of the word joints
- Intents is a particularization, definition, or expansion of the word thoughts
And, because 12b appears to be a pre-literary creed or hymn, in poetic parallelism, we can also say:
- Joints is a particularization, definition, or expansion of the word soul
- Thoughts is a particularization, definition, or expansion of the word joints
- Marrow is a particularization, definition, or expansion of the word spirit
- Intents is a particularization, definition, or expansion of the word marrow
Therefore, we can also conclude:
- Both joints and thoughts are a particularization, definition, or expansion of the word soul
- Both marrow and intents are a particularization, definition, or expansion of the word spirit
And finally, we can say:
- Spirit, marrow, and intents as category B, are a particularization, definition, or expansion of category A: soul, joints, and thoughts
Therefore, to answer the original question, soul can be understood in terms of joints and thoughts, and spirit can be understood in terms of marrow and intents, even as soul in general, can be understood in terms of spirit.
So, what are joints? Thoughts? Marrow? Intents? And how do these inform our understanding of the two terms in question?
A joint is the location where two or more body parts intersect and connect. Thoughts are the ideas, concepts, and images that pass through the human mind on any topic whatsoever at any time whatsoever. Marrow is soft tissue found inside of bones--connected to each other at the joints--where life giving red blood cells, most white blood cells, and platelets, are created by the body. And finally, intents are whatever things the mind purposes to do in order to bring about the realization of every idea, concept, or image as contained in one's thoughts.
Using these simple definitions, we can then say of soul:
In some way, a soul is an intersection and connecting location, where ideas, concepts, and images all come together in the human mind.
In some way, a spirit is where life is given to the human body and mind in order that they may exist to purpose and do whatsoever a human body and mind are capable of purposing and doing.
Additionally, following Kugel and Alter, a soul is also the locus of the human body and mind, the very center of all life's activity, where a human initially engages every idea, concept, or image that comes his or her way.
Would the first readers of the Epistle to the Hebrews have understood all this?
I believe so, yes. Here's why: As Hebrews, Jews specifically, they would have been very familiar with Biblical parallelism. They would have recognized the poetic form, the dichotomous play between soul and spirit, joints and marrow, and thoughts and intents.
Having been rather learned in the Old Testament (clearly presupposed by the author of the Epistle), the initial recipients of Hebrews would have already been exposed to the idea of how, in a tristich, as Kugel argues, B would inform one's understanding of A, not as "mere restatement", but as a particularization, definition, or expansion.
For example, from Psalm 120:2,
Save me, O Lord, from lying lips
and from deceitful tongues.
The phrase "deceitful tongues" is not just a restatement of "lying lips", but helps particularize, define, and expand on what the phrase "lying lips" means. In this instance, lying lips do not merely represent someone who accidentally passes along false information, but rather, describes the actions of someone who intentionally desires to deceive and trick his or her audience. This speaks to malice and immorality.
This being the case, it's clear the original readers would have had no trouble at all seeing how words like spirit, marrow, and intents, arranged as they were and are, not only undergirded, but also built upon the concepts represented by the words soul, joints, and thoughts.