What Governs Human Behavior

By Ben Suggs, LPC-A - City Center Counseling Ministry

Recently I was reading a Substack article titled 4 Reasons Why You Should Get a Tattoo (From a Christian) by Daniel Weaver - anyone that has met me would understand why I might be interested in this topic. At one point in his article, Weaver is commenting on the sense of commitment that tattoos evoke and how committing to something one might not be as “stoked on” several years later (like a tattoo) is a good life lesson. He then references The Screwtape Letters by C.S. Lewis, where Lewis describes this idea known as “The Law of Undulation” - here is how Weaver describes it.

“This is described by C. S. Lewis as “The Law of Undulation,” that our feelings about certain things constantly ebb and flow and that it’s normal. The trick is, what are you going to do as your feelings toss to and fro? Are you going to stop committing to good things because they may not feel as nice as the day you start them? Are you going to let your feelings determine your commitment?”

A common mistake that we make is believing our feelings are supposed to stay consistent. When our motivation fades or our emotions shift, we assume something is wrong—with our friendship, ourselves, or our marriage. C.S. Lewis argued the opposite in The Screwtape Letters. He believed fluctuation isn't evidence that something is broken; it's part of what it means to be human. “As spirits they [humans] belong to the eternal world, but as animals they inhabit time. This means that while their spirit can be directed to an eternal object, their bodies, passions, and imaginations are in continual change, for to be in time means to change.”

This can be an extremely frustrating conclusion, but this is also a case of perspective, maybe there is more freedom in acceptance of undulation rather than trying to escape it. Death, taxes, and human beings living in the effects of the Fall - we cannot escape undulation. Perhaps the ebbs and flows of our humanity can be a teacher rather than a torturer - what can we learn from undulation?

Let’s look at a few scenarios of this Law in practice.

Imagine you’re spending time with a good friend and they make a comment that really upsets you - maybe they trash your favorite sports team, or they’re not really paying attention to you when you’re speaking, and you start to wonder how they feel about you. Does my friend really love me? Have I invested all this time in the wrong relationship?

Or maybe you have a consistent workout routine established in your life, it might not be your most favorite time of the week but you have been able to see the fruits of your labor. Then one random Monday morning, perhaps for no reason at all, every reason why you would want to go to the gym is no longer there - and all you feel is dread at the thought of walking through those gym doors.

This fear of change can creep its way into even our deepest relationships, maybe your marriage is experiencing its own rough patches and that emotional “ebb” starts to weigh heavily. Because of the weight of this mental valley, it might feel more difficult than usual to love your spouse in these moments.

In these scenarios, friendship, exercise, and marriage are objectively good standards that we are drawn to. If, as Christians, we are to believe that everything that is good comes from God, then one can confidently say that good health, union, and community is also a gift from God.

Within the world of Psychology, this Law can fit into several different terms - For the sake of this article, The Law of Undulation can best be looked through the lens of Acceptance and Commitment Therapy (ACT).

“Acceptance and commitment therapy (ACT) is an innovative psychological intervention emphasizing psychological flexibility and values-driven actions to enhance overall well-being. … ACT encourages the acceptance of negative thoughts and emotions while fostering a commitment to personal values.”

Anusuya, S.P., Gayatridevi, S. “Acceptance and Commitment Therapy and Psychological Well-Being: A Narrative Review”, 2025 doi: 10.7759/cureus.77705. PMID: 39974259; PMCID: PMC11837766.

ACT strikes a strong similarity to Lewis’ Law of Undulation - acting according to one’s values even when emotions fluctuate. I also want to highlight that last section of the quote, “the acceptance of negative thoughts and emotions while fostering a commitment to personal values.” We as humans are allowed to accept the fact that we are going to have negative thoughts and emotions, while simultaneously, adhering to our own values. This is an important lesson that I was taught early on in my supervision training - that two thoughts, desires, values, etc. that seem inherently opposite can exist within ourselves, and can also teach us a lot about ourselves.

As a (proposed) Law of Human Nature, this means that anyone can experience an ebb and flow at any point in their life - our lives are inevitably subject to change, even while our deepest commitments can remain fixed. Where people might get hung up is the shame associated with undulation - no one wants to admit that they’ve experienced it. The good news is that undulation does not dictate what we decide to do with our emotions - for example, we still have the choice to love our spouse in spite of our brokenness wanting to tell us otherwise. For Christians, we have a prime example of someone choosing love in spite of feeling like doing the opposite - Jesus in the Garden of Gethsemane.

If anyone had a solid reason to let their emotions dictate their actions, it would be Jesus in Gethsemane. Scriptures tell us he was deeply grieved to the point of death. Yet, in the face of this tidal wave of emotions - Jesus is able to remain steadfast in his conviction to the Father. His feelings and obedience were able to exist within the same being.

Then Jesus came with them to a place called Gethsemane, and he told the disciples “Sit here while I go over there and pray.” Taking along Peter and the two sons of Zebedee, he began to be sorrowful and troubled. He said to them, “I am deeply grieved to the point of death. Remain here and stay awake with me.” Going a little farther, he fell face down and prayed, “My Father, if it is possible, let this cup pass from me. Yet not as I will, but as you will.”

Matthew 26:36-39 CSB

So what can The Law of Undulation teach us? - The Law of Undulation reminds us that feelings tend to make poor foundations. Our thoughts and emotions can change like the phases of the moon, we might fluctuate and have undesirable thoughts. Love, faithfulness, and conviction, however, were never meant to depend on emotional certainty. We are free to acknowledge every emotion we experience without allowing those emotions to determine who we become. Just because an undesirable thought feels like truth, does not mean it is truth. Think of your emotions as loud children on a long road trip - you can’t leave them in the trunk, as much as you might want to, but you also can’t let them drive the car. We must remain in the driver’s seat while learning to coexist with our changing emotional states. The question is what will be in the driver’s seat if our emotions fluctuate - our conviction to our values or the shifting sands of our emotions?