I know: a title like this is more appropriate for a book than a blog post. We will only be scratching the surface here.
“Flesh” (σαρξ) is a term intimately connected with the idea of the body (σωμα). Loosely speaking, we might say that “flesh” is the substance of which the body is composed. A man is composed of spirit (πνευμα), soul (ψυχη), and body (σωμα). The body has urges, needs, desires. The continuation of existence, marked by pleasure rather than pain, and the need for procreation mark basic outlines of the body’s principles. Our attempts to meet these needs produce works such as: housing; agriculture; dating; marriage; medicine; sanitation; trade; and so forth. (N.B. I am not taking a materialist line here.)
The soul, while able to do much in the abstract, is to a great extent trained to think through the medium of physical life, which the body with its senses enables. Indeed, the senses may be understood as a conjunction of physicality and mind; to say “That feels hot”, is to interpret data provided by means of the body or other sensory equipement. (For further philosophical discussion of perception, see Russell, Berkeley, Locke, Hume, and Leibniz.) While some things that the body needs and accomplishes can be experienced without sophisticated conscious thought, a great deal is done by means of the conscious mind.
This foundation enables us to understand how Paul can write of the flesh as something with “desires” and thought-forms (“carnal thinking”), and yet as a “house” or “tabernacle” in which the non-tangible aspects of humanity dwell (viz. the soul and spirit). Indeed the complexity and unity of the human person make it difficult to determine the kind of language one should use in describing certain states – particularly from a Christian mindset. “I am hungry”? “My body is hungry”? “This body is hungry”? Furthermore, inasmuchas the Holy Spirit dwells in each believer, His desires are also “perceived” by the mind, in many cases such that they seem to be the desires of the human. Indeed, one might rightly say that they are the desires of the human being, inasmuchas a person who is aligned with God would desire the same things that God would desire (e.g. justice), even though such things might not seem desirable from the perspective of personal, physical comfort.
Paul tells the Galatians that the flesh and the Spirit are at war with each other. He advises his readers to learn to be “led by the Spirit” so that they will not satisfy the desires of the flesh. From an ethical perspective, we may analyse the “desires of the flesh” that are understood as “wrong” in this passage as falling into at least two groups:
A) Things that are always wrong (wrong by their very nature);
B) Things that are wrong owing to particular conditions.
Murder is an example of Category A: it is wrong by its very nature (as opposed to justified killing – e.g. in war). Over-eating is an example of Category B: eating in principle is not wrong (indeed, it is necessary for life), but eating to excess is a distortion of what eating is meant to be.
These supplementary thoughts are useful in helping us to understand Paul’s qualification of the flesh by means of the phrase “with its sinful desires”. They also underpin the need to be “led by the Spirit”, as he urges the Galatians. The ideal state is not doing nothing; rather, it is acting in accordance with the will of God, in general and specific terms.
The flesh, according to such a system of thought tempts the will to make decisions that are wrong. The flesh may be understood as inherently selfish, according to the understanding of sin as rebellion against God, “going my own way”. Its desires are predicated on satisfying itself first, which is why self-denial and self-restraint are inimical to it. Furthermore, discernment is necessary because our motives are not always clear, which is one of the reasons Paul admonishes Christians to be “transformed by the renewing of the mind”. It is possible to think that one’s actions are motivated by a moral principle of meeting the needs of others, when really they are driven by a desire to protect oneself from an alternative that is considered more harmful than the “moral” option. Indeed, the Scripture says, “the heart [sc. mind and emotions] is deceitful above all things…who can know it?” (Jer. 17:9)
Careful reading of the Greek reveals different nuances behind the terms “body” and “flesh”, with shifting meanings according to their different contexts. One extreme shows no respect for the human body at all, while the other seeks to satisfy its every whim. Paul saw no problem with borrowing from the Greek culture that surrounded him. When it came to “living a good life”, he was very happy to import the concept of σωφροσυνη – soundness of mind, temperance.