"The spirit is willing, but the body is weak." (Matthew 26:40-41)

Then he returned to his disciples and found them sleeping. "Couldn't you men keep watch with me for one hour?" he asked Peter. "Watch and pray so that you will not fall into temptation. The spirit is willing, but the flesh is weak." (Matthew 26:40-41)

Why did Jesus say this?

Jesus said this to Peter, James and John after his first prayer at Gethsemane.

Jesus was making two important points here. The first is that his disciples had not followed his instruction. Jesus had requested that Peter, John and James keep watch, as he was expecting to be arrested shortly:
"Stay here and keep watch with me." (Matthew 26:38)
So in the first part of Matt. 26:40 above, Jesus is chastising some of his disciples for not following his instructions to watch out for him while he prayed.

The second part of this statement indicates a core element of Jesus' teachings. Jesus clearly taught the  distinction between the spiritual person (soul or spirit-person) and the physical body:

What does 'the spirit is willing' mean?

Who is Jesus referring to as the "spirit"? We can see here that the "spirit" has a will - "is willing." Only a person can have a will, and thus we can conclude that the "spirit" Jesus refers to is the person within.

The word "spirit" is being translated from the Greek word πνεῦμα (pneuma). According to the lexicon, this means, "the vital principal by which the body is animated." as well as "a life giving spirit," and "the soul."

Many refer to the soul as a part of a person - like an organ or something. Others will refer to the soul as one's morality.

Here we can see that Jesus' use of the word "spirit" does not relate to a part of a person or one's morality. Jesus is referring to "spirit" as the individual spiritual person: Again, only an individual person (spirit) can have a will.

What does 'the body is weak' mean?

But then the verse states Jesus said, "but the body is weak." This is a slight mistranslation, because the Greek word being translated to "body" is σάρξ (sarx), which according to the lexicon means "flesh (the soft substance of the living body, which covers the bones and is permeated with blood) of both man and beasts." Without the parenthesis: "flesh of both man and beasts."

More importantly, the word also means, again according to the lexicon, "the sensuous nature of man, "the animal nature"" and "the animal nature with cravings which incite to sin."

We can see from these definitions that the word "body" falls short of capturing the real meaning of Jesus' statement. He was saying that while the spiritual person might be willing to do something, the cravings of the physical body can pull the person otherwise.

Why is the body 'weak'?

How can we be willing to do something, yet be driven to do something else by virtue of our physical body and its inherent "cravings"? Many characterize this as "the devil," as in: "the devil made me do it." This is often conjured as having a little devil on our shoulders, who is constantly tempting us.

The answer lies within the reason for our occupation of these physical bodies in the first place.

Each of us is a spiritual individual created by the Supreme Being to share the spiritual realm with Him, play with Him, and be one of His care-givers. He wanted to share loving relationships with us. But real love requires freedom, so God gave us the freedom to love Him or not. He gave us the choice to engage in a relationship with Him or not.

Some of us decided to exercise that freedom. So we "ate the fruit" of self-centeredness, and decided we did not want to be His caregiver anymore.

Since the population of the spiritual realm is full of love without self-centeredness, those of us who chose not to love Him were relocated away from the spiritual realm.

Since we are spiritual by nature, this relocation requires a virtual existence. One where we could take on a role different than our real nature. In order to take on a self-centered existence, we need a virtual personality - because our true nature is loving and selfless.

So the Supreme Being designed a virtual universe where we could occupy temporary physical bodies and play temporary roles, in order to exercise our self-centeredness.

We might compare this to a play, where the actors put on costumes and play roles different than their "real" roles.

To complete the analogy, our "costumes" are these physical bodies.

We know these physical bodies are temporary and we are playing temporary roles in them because our bodies keep changing and our roles keep changing - yet each of us is still the same person within. When our body is young, we play the role of a child. When our body grows older, we play the role of a teenager. Later we play the role of an adult, with a job and family. Then later we play the role of a senior citizen.

Each of us is distinct from the roles we play in the physical world. These roles are required by the society and culture of the physical world. 

This doesn't mean that we have to identify with these roles, however. We can know that when our physical body dies, our roles in this world will die with it.

Why does the body not satisfy us?

The physical body is a temporary vehicle for the living being. Each of us is driving our physical body much like a person would drive a car. And as most of us have experienced, when we are driving a car we begin to identify with it.

We also begin to identify the other drivers on the road with their cars. When we are passed on the highway by a speeder in a red car, we will say, "that red car is speeding." We are thus identifying the driver as their car.

In the same way, after a short time within these physical bodies, we begin to identify with them. This is the design of the physical world, as programmed by the Supreme Being.

As we identify with these physical bodies, we also seek our happiness through them. We begin to think that our happiness revolves around whether the body feels good. This is also designed by the Supreme Being.

The body is designed to release neurotransmitters called dopamine, serotonin and endorphins, which signal to our brain that something is good for the body. Eating something sweet or having a sexual orgasm, for example, will cause the brain to release these "feel good" chemicals. These "feel good" chemicals trick us - the spirit-person within - into thinking we become happy when the nerves are saturated with one or more of these "feel good" neurotransmitters.

Yet no one actually becomes happier when the body is saturated with these neurotransmitters. The sensation might feel good to our body, but the spiritual person within is not satisfied.

This might be likened to a car driver feeling hungry and thinking that if he filled his car's tank with gas the driver would not feel hungry anymore. No. The car's gas tank gets filled up, but the driver's stomach is not touched by the gas. The driver will still feel hungry, even when the gas tank is full.

It is the same with the spiritual being driving the physical body. The reason that so many people living in the physical world are unhappy - even those with great wealth and all the sweet foods and sexual organisms they could ever want - is because we are spiritual in essence, not physical.

And this is what Jesus is trying to teach his students. His students' bodies may feel more comfortable sleeping, but their spiritual selves will only be satisfied when they are fixed within their real identity - being one of the Supreme Being's loving servants.

The Supreme Being wants us back. He wants us to return to His loving arms. This is why Jesus' most important teaching was:
"'Love the Lord your God with all your heart and with all your soul and with all your mind.' This is the first and greatest commandment." (Matt. 22:37-38)