(Note: Please refer to the previous two posts, as they relate to this one.)
St. Thomas Aquinas, in his Summa Theologica, he describes the vice as the "sorrow for spiritual good", but also explains why it is a capital sin, and not to be confused as something of just a feeling or less dangerous. One might say wrongly that sloth moves one to inaction, and therefore there is no sin to which it is motivated, but Pope St. Gregory listed "daughters of sloth", from which St. Thomas Aquinas primary draws. They are:
Sluggishness in regard to the Commandments
Wandering of the mind after unlawful things
Before I go through which each of these mean, let's first define what we mean by 'spiritual good'. Since we are here to know, love and serve God with our whole heart and whole mind and being, then we can see that a spiritual good can be more than the liturgy, private prayers, and spiritual readings (etc.), but seeing all daily avoiding sin, whether great (grave) or small and the pursuit of greater virtue. Furthermore, the end to which these are to be pursued is for God and God alone, rather than for some feeling or any other gain.
St. Thomas says that the two steps by which someone fails to regard an unpleasant good are:
1.) First, he comes to withdraw from, or avoid, the good itself,
2.) Then, he seeks for something else more agreeable. In other words, he struggles actively against having to perform the higher good in question.
Malice is one of the daughters of sloth because it is the result of the habit of preferring one's will over God's to such a degree that the perceived arduous good is one that threatens the ease or lesser good that the person prefers. Quite simply, it is malice because one becomes angry to think that they should have to do that which has become so distasteful to them, even to the point of detesting that good. Examples: Hating a liturgy that looks only to God for looking only to God, or detesting to put God first in daily priorities to the point of having to put one's wants last over another needs for whom we are responsible (i,e.: our child, or spouse).
This is the avoidance of spiritual good that is the reasonable means to the end in matters of genuine difficulty. One example is avoiding deliberate unnecessary near occasions of sin. This is always a bad thing, but in the faint-hearted, often they will even make excuses to not avoid the near occasions that lead them to their chief vices, whether they are mortal or venial sins. Just because something is difficult, doesn't mean that you should not own up to your responsibility. Jesus, God the Son, suffered and died on a Cross, so that your crosses for what you are Commanded to do are bearable through grace (that he won for you). To say otherwise is a lack of humility (*to see oneself in the light of who God is), because He gives us the means to overcome temptation, if we are close to the Blessed Mother and "do whatever He tells you." Jesus, who commands that we "be perfect, as your Father in Heaven is perfect," does not command the impossible, but it is we who, in faint-heartedness, lose faith by not living by it in charity.
Ultimately, the more one becomes faint-hearted, the more one deeply descends into the vice of sloth, they not only weaken their faith, but they lose hope in the things of Heaven, and the promises of the helps of God to get to Heaven. Despair is then the avoidance of spiritual good considered as an end in itself. Since sloth tends towards one's feelings, whether one reasons these feelings are a priority or not, the deeper descent into this vice often leads to sins of lust, which further ties one to this sin. Also, since one despairs of the spiritual goods as ends in themselves, to give glory to God in gratitude to Him, for it is our duty to know, to love, and to serve Him (it's why we are here), one despairs, inevitably of the promises that God makes to those that do serve Him in this life.
Sluggishness in the Practice of the Commandments.
This is the avoidance of spiritual good that is the reasonable means to the end in matters of common righteousness. Even private sins affect the world, because they create disorder in the soul, and in the resulting actions borne of the wounds from those effects in the soul, heart and mind of the sinner. The person with this daughter of sloth doesn't look to observe the commandments closely, with eagerness and faith that they will overcome their most deeply seeded vices by the grace of God (and prayer of the Blessed Virgin), and working towards constancy and consistency, because they have chosen to perceive the pursuit (of the Commandment in question or Commandments in general) as either looking too closely at the obligation, or because they want to procrastinate their observance.
(Incidentally: The suggestion that the practice of the commandments "paralyzes" one from their freedom is a great example of sloth, but of a different daughter. It encourages the daughter of sluggishness in the practice of commandments by the the daughters of malice and spite. Freedom is ability to chose the higher good, and graces are the means to obedience. We should follow them, less we lose them and offend God who gave us His only son to die on a cross as a means of giving us graces. Obedience to God IS the measure of gratitude to Him.)
Wandering of the Mind to Unlawful Things
This daughter is the last step which the vice of sloth generates over vices, where we pass over the arduous or unpleasant good in favor of something easy and pleasant, simply because they find no joy in spiritual things. Now, this means in context within one's state of life, and takes into account those duties and with their priority, with our reason for being (the eschatological realities) as our final end. By that, even in the smallest duties that are required of our state may be prioritized and not ignored, but done with God as our devotion in their accomplishment, and both with Him and by Him, that they be for Him. If we dissipate our time, rather, in frivolous pursuits, or sin in loquacity for sake of avoidance of spiritual good, even unconsciously but out of habit, or if we simply allow ourselves to be distracted at Mass or in private prayer, these are all examples of "unlawful things". Someone might say, "Well, having a conversation is not 'unlawful'!" Well, it itself, of course not, neglect of duties due to long conversations would be giving in to a "wandering of the mind". Often, the mind might be accustomed to wandering in prayer, or leaving it off to a point where one is more inclined to not being as mindful. You can see how easily the mind becomes trained to this vice, as one takes his mind and heart off of God, who never-the-less, is always seeing everything we are doing or thinking in every given moment.
I hope this makes more sense. There aren't a lot of resources on the web for this, but you can read St. Thomas Aquinas's Summa Theologica: The Vice of Sloth online.
A few final words on this subject:
Sloth is the sorrow for the spiritual good, and what is sorrow? But it is the absence of joy, so that is why often when we commit this sin, we EXAGGERATE the effort required for the spiritual good. Why do we do this? We do this because of a LACK OF FAITH. The easy solution to this is to ACT OUT OF FAITH. Our Lord tells us (and shows this all the time, since He is never outdone in charity, since HE IS the Creator of all good), that we only need a "mustard seed of faith" with which to begin. (Notice: I said, "begin", not end and/or sit there and say that a "seed" should remain just a "mustard seed". Remember the Gospel reading for Sextagesima Sunday (according to the traditional cycle based on the Breviary), Luke 8:4-15. Actions based on faith are essentially planting the seeds of grace that were given to us by God. They are, as a good canon once put, and I love this, "GOLDEN OPPORTUNITIES FOR GRACE!" So, unlike any other kind of commerce, the more we SPEND this well, the more of this we receive back. It's like, imagine going to a store to purchase something you need. You pay the total for the bill, and then you are handed back all that you just handed over and a whole lot more. That's a great deal! I DARE YOU to find a better 'bargain' anywhere! You certainly won't find a better 'commodity' in this life.)
So let us not be slothful in winning the war on sloth within our souls. We start with our minds and our hearts will soon follow. We will very soon be given the GREAT JOY, that increases beyond imagination, and is constant and consistent to the degree that we remain constant and consistent in pursuing the virtues for the love of God, who Loves us first and MADE us that we *might* give HIM glory by turning our self-inclined wills over to His will. For He alone knows what will give us the deepest and greatest happiness, and the kind that can never be taken away or damaged, if we hold onto it tightly with both hands, with our practice of overcoming vice. Let us even enjoy the struggle, as we battle, "not as though beating the air" but TO WIN the award, and in the end, an AWARD SO GREAT, that we cannot imagine. Feeling still overwhelmed? Don't be, because the battle is now. The adventure is not one you pursue: you're in it. You just have to decide whether you're going to float like a dead fish on the tides, or fly soar like an eagle in the clouds. The Commandments are not just rules, they are our guide, and within each of them we can find every virtue. This is your game plan, God's means to the path HE has chosen from your very beginning, as He knew you even before you were born.
You've got this; now GO TO IT!