Sexual Sin Makes You Dumber

St. Thomas Aquinas on the Daughters of Lust: Part 1

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This is the first installment of a series of 2 articles titled St Thomas Aquinas on the Daughters of Lust: A systematic breakdown of how lust affects the will and intellect.

Part 2 drops the 4th of June, 2026.

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A common misconception that keeps many enslaved to sexual sin is that they think that it’s exclusively a private matter. They believe that lustful actions do not cause any harm beyond the individual spiritual life, and thus, on some level, they think it’s “not that bad” to engage in sexually disordered actions.

The problem with this view is that it ignores the very tangible consequences that persistent sexual sin has on the will and intellect. St. Thomas Aquinas, in his Summa Theologiae, identifies what he calls the “eight daughters of lust”¹, eight negative effects that sexual immorality has on the will and intellect. Four of these “daughters” correspond to the intellect, and four correspond to the will.

Anecdotically, it’s easy to see the truth in Aquinas’ theology: you have probably encountered at least some men who are intelligent, capable, well-intentioned and otherwise disciplined, but whose judgment and willpower collapses consistently in certain areas of their lives, usually when it comes to acting chastely or controlling their sexual urges. These are men who seem to have their lives together, and yet, when it comes to sexual immorality, they have some pitch black blind spots that lead to them justifying their sin or simply ignoring the incoherent way in which they are behaving.

These blind spots are caused by the four daughters of lust that affect the intellect, which we’ll analyze in this article. In the next installment of this series, we’ll take a look at the remaining four daughters, which are the ones that affect the will.

The goal of this series is to help you run a more honest and unbiased self-diagnosis, and to help you understand just how deeply sexual sin affects you, so that you will have more reasons for fighting against it. Towards the end of this article, I’ll also share some practical resources to defeat pornography and other forms of sexual immorality.

Lust Is Not a Private Sin

As mentioned earlier, it’s important to begin by understanding that sexual sin is not private, in the sense that it’s not self-contained. It’s not just a personal weakness that doesn’t bleed into the rest of your life. The premise that defeats this assumption is that a human is an integrated whole, and just like your physical health affects your mental health, so too spiritual disorder causes intellectual disorder, for example. In that sense, we need to understand that the will and the intellect are not isolated faculties —and no section of your life is completely isolated.

Disorders in one faculty or area create more disorders in other areas. Sexual sin doesn’t “stay in its lane”, it wrecks havoc in all other areas of your life, by clouding your intellect and lessening your willpower.

The specific way in which lustful actions affect the intellect is as follows:

The 4 Daughters of Lust that Affect the Intellect

These “four daughters” are consequences that persistent sexual sin has on your intellect. Think of them as blind spots that appear when you keep engaging in sexually immoral behavior.

• Blindness of mind: This is defined as the inability to think with clarity about God, one’s own life, and one’s obligations. Blindness of mind is the effect by which the right knowledge of spiritual things is prevented. The man who cannot pray seriously, for example, is often not spiritually dry but sexually disordered.
  Example: A man who habitually consumes pornography can still “know” religious facts intellectually, but in practice he becomes incapable of perceiving the deeper realities of what grace, repentance, or chastity actually means for him. The consequences of this is that prayer feels flat, Scripture doesn’t penetrate, and even serious spiritual warnings seem unreal or irrelevant.

• Rashness: This is a collapse of deliberation, or, in plain terms, acting without thinking. This occurs because there is a connection between habitual lust and impulsive decision-making in other areas of life. After all, a man who can’t control his sexual impulses will be less capable of controlling his other impulses, making him more emotional, reactive, and prone to acting without weighing the consequences of his actions.
  Example: A man sees a “quick” path to gratification (a message, an unsupervised situation, a click he tells himself is harmless) and moves immediately, without consulting conscience, prayer, or seeking counsel, because the potential pleasure has already fixed his attention and weakened careful judgment of what spiritual good is at stake.

• Thoughtlessness/lack of consideration: Thoughtlessness is similar to rashness so I’ll do my best to explain why it’s distinct from the other daughters of lust. Rashness means acting impulsively without allowing space for deliberation, whereas thoughtlessness means a lack of intentionality in your choices, failing to examine the consequences rightly and an inability to properly weigh the importance of the matter. Think of it this way: rashness is when you hit click on an immoral picture right away before counsel and deliberation; thoughtlessness is when you don’t even consider the consequences properly in the first place. They are closely linked together, but they are fundamentally different.
  Example: After noticing an urge or temptation, a man doesn’t slow down to examine what’s happening (What am I choosing? What will this lead to? Is this compatible with virtue and his duties?). He fails to reflect because his intellect is drifting.

• Inconstancy: Inconstancy appears as a vice similar to effeminacy, in the sense that it is an incapability of persisting in what sound reason has chosen and abandoning it under pressure.
  Example: A man makes a good resolution or commitment: “I’m done; I’ll never go back to watching porn,” but when the next strong trigger arrives (stress, loneliness, late-night phone use), he ignores the earlier judgment and returns to the sin, not because he changed his mind about what was right, but because the lust-impulse overpowers the decision he previously chose and understood.

As you can see, lust is not something “harmless” or isolated. Sexual immorality is, in fact, a vice that clouds your intellect, makes you less capable of reasoning properly, and put bluntly, makes you dumber and blind to the consequences of your actions.

It’s quite interesting, when you think about it: why do you think that so many obviously erroneous ideologies are so widespread today? Why do you think that there is so many people who seem to have no common sense?

As we’ve seen, in a culture in which sexual immorality is the norm, the natural —and expected— consequence is that people’s intellects will be clouded, and thus they will lose the ability to reason properly, becoming more susceptible to propaganda, lack of emotional self-control, and thus it’ll be easier for dangerous ideologies to spread.

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The Good News

I’m not sharing all this to try and make you feel guilty or hopeless, but simply as a theological and psychological theory that will help you in your fight against sexual immorality. In fact, I believe this should, if anything, increase your hope, because now you see just how much better your life will be after quitting pornography and sexually immoral behavior. By understanding that your intellect is clouded, you can now see how much brighter a life of chastity is.

Because the good news is that if lust produces these effects systematically, then chastity reverses them. When you start practicing chastity and purifying your life, you’ll gain the opposite virtues to the vices that lust causes: understanding (the virtue opposite blindness of mind), counsel (the virtue opposite rashness), sound judgment (the virtue opposite thoughtlessness), and constancy (the virtue opposite inconstancy).

Gaining custody of your sexual life does not merely make you feel better, it —literally— makes you smarter! Your intellect clears. Your judgment improves. Your prayer life becomes possible again. You gain wisdom and understanding.

The same faculties that were degraded by the vice are restored by the contrary virtue.

On a Practical Note

We’ve seen just how deeply sexual immorality affects you and how important it is to purify your intellect. I can also tell you, from experience, that the theory we just examined is undeniably true. My life has changed dramatically ever since I started living chastely. It’s one of those things that you likely won’t even believe until you’ve experienced it yourself, but it’s as if you had been living with a veil over your eyes that lifts as soon as you clean up your life, allowing you to see the world clearly and understand profound things better.

If you want to think more clearly, lead more effectively, pray more seriously, and govern your household with authority, quitting porn and sexual immorality is the place to start. Not because sexual purity is the whole or end goal of the Christian life, but because its absence makes the rest of the Christian life progressively harder to live.

Practical Resources to Help You Quit

I’ve written lots of articles about quitting sexual immorality in this blog, which I’ll link at the end of the article, but to save you some time, the main resource I always recommend is Relay, a Christian app designed specifically for overcoming pornography and sexual sin. It provides structured accountability, evidence-based protocols, and a community of men fighting the same battle.

There’s a ton of positive testimonials of men who have found freedom from pornography addiction, masturbation, and other forms of sexual immorality with the help of the structured, evidence-based protocol that Relay offers. Many men (even some close friends) have also told me personally it made the difference after other methods failed.

If you have been trying other methods and they haven’t worked, I recommend you give this one a try and see if it helps you. You can get a 7-day free trial with the code AETERNUM:

The Man You Could Be

Maybe after reading this you notice some of the blind spots that sexual immorality has caused in your own intellect, or maybe you don’t. Whatever is the case, I want to encourage you to genuinely pursue chastity. The clean, sharp intellect that chastity produces is one of many reasons why you should absolutely prioritize quitting pornography, masturbation, and sexual immorality as a whole, but it’s not even the most important one.

We live in a spiritual universe, in which our actions have more consequences than we can see with our eyes. Every single evil thing you do has a spiritual consequence, beyond the psychological consequences that sin also carries with it.

This is why Christ warns us so strongly against sins of the flesh:

If your right eye causes you to sin, pluck it out and throw it away; it is better that you lose one of your members than that your whole body be thrown into hell.
— Matthew 5:29

No sin is “meaningless”. No sin is private. Sexual sin definitely isn’t. We need to face the harsh reality of the evil it contains, because only then will we understand why it’s so critical to quit.

Beware, brother, if you do not be converted now, you may never be converted.
— St. Alphonsus Liguori

Do not postpone this. Do not brush it off. Take it seriously. Do whatever you need to do to purify your life. Work to order your life towards all that’s good, beautiful, and true, and be radical in rejecting sin. Give it all to Christ, and let Him be your strength. Help Him too, by making an effort, by taking this purpose seriously, and by doing whatever it takes to get rid of these sins that have led so many men to intellectual disorders, but mainly —and much worse— to eternal damnation.

God is with you, you are in my prayers, and surely too in the prayers of all the men who fight this battle alongside you.

May God bless you and keep you.

Ad Maiora Nati Sumus,

Juan

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PART 2:

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Additional Resources

The Best Protocol For Quitting P*rn

The 5 Levels Of Lust

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1. ST II-II Q. 153, A. 5.