There is no science of particulars, for science is the result of demonstration (syllogism), it is essentially certain knowledge attained by seeing things in relation to their causes. Now it is the middle term that represents the cause in a demonstration, for the middle term is the cause of the conclusion. For example,
All men are rational
John is a man
Therefore, John is rational.
There is no conclusion "John is rational" without the middle term "man". "Man" is the cause, the principle upon which the conclusion depends; it is the reason why we can conclude with necessity, that John is rational - because all men are rational and John is a man.
Note also that no necessary conclusion can be drawn from two particular premises. Hence, there is no science of particulars. At least one premise must be universal.
What does all this mean for ethics? Ethics is a science, and so the science of ethics aims at universal conclusions: i.e., Injustice is a vice, lying is unjust, man ought to be temperate, intentionally killing another is always wrong, etc.
Now the virtue of prudence involves applying universal moral principles to particular situations. But particular situations are unique and unrepeatable. Indeed, situations can be similar to one another, which is why we often hear the following prelude: "…if you ever find yourself in a similar situation, this is what you ought to do…" However, although situations might be similar in many respects, they are never identical. That is why Aquinas speaks of the quasi-integral parts of prudence, which are: memory, understanding, docility, shrewdness, reasoning, foresight, circumspection, and caution. Understanding of first principles and the ability to reason are necessary if one is to apply universal principles to particular situations, but memory, docility, shrewdness, foresight, circumspection and caution are necessary precisely because prudence concerns itself with particular situations, and particular situations are unique. Thus, one requires experience, the ability to learn from the experienced, and the foresight that comes from many years of experience, as well as circumspection and caution, which are missing in the inexperienced.
In other words, although it is of the utmost importance to study the science of ethics, a textbook is not going to be much help when you find yourself in the midst of particular situations that demand a response - running to the library will not work. It is going to be up to you to "incarnate" the moral truth within the heart of the situation you find yourself in. To do that is no easy task. It requires these quasi-integral parts of prudence, and it requires the cardinal virtues of justice (which is in the will), temperance (which is in the concupiscible appetite) and fortitude (which is in the irascible appetite).
In other words, one needs more than the ability to reason. Unless all we want to do is discuss abstract moral problems and are indifferent to living and incarnating the truth in our own life, we need connatural knowledge.
In order to make some sense out of this, let me begin by defining prudence: it is the intellectual/moral virtue which rightly directs particular human acts, through rightly ordered appetite, toward a good end. The key phrase here is "rightly ordered appetite". A prudent man is one who has a rightly ordered appetite. In other words, a person who is disordered, who is unjust and lacking temperance, for example, cannot be said to be prudent, although he might be a great physicist or a good speculative philosopher. A prudent man is a virtuous man (one who makes moral truth a lived phenomenon). If a person makes imprudent decisions - and immoral choices are imprudent choices - , he is not prudent.
Now, prudence is the mother of all the virtues. But how can I be rightly ordered unless I have prudence? And yet how can I possess prudence unless I possess right appetite? These are very good questions and they are not easy to answer without appearing to be caught in a vicious circle.
First, let us review connatural knowledge. Connatural knowledge is the opposite of abstractive knowledge. For example, all knowledge begins with sense experience. I see a particular thing, i.e., an animal, and I observe its behavior, and through its activity I gradually come to know "what it is" (its essence or nature). My mind conceives an idea of it through a process of abstraction. And so the word "dog", "cow" or "man", for example, are words that point in two directions; they point outwards to a particular dog, cow, or man (i.e., that is my dog), and they point back to a universal idea in the mind (canine, bovine, human, etc.). So the movement here is from the particular (this dog) to the universal idea (canine).
For the sake of clarity and simplicity, let's just say that connatural knowledge begins with the idea and moves to the particular (it really begins, however, with the particular). A connatural knowledge of justice, for example, is a lived knowledge of justice. The idea of justice has been incarnated into my very person. I know justice from within, because I am naturally inclined to justice, because I possess the habit or virtue of justice. A connatural knowledge of chastity, for example, is a lived knowledge of chastity. I know chastity from within, I know that this option before me is contrary to chastity, because I am disinclined to go there, but I am inclined to that option, because the virtue in me inclines me to that option (for example, I am not inclined to go to a Swingers club or watch pornography, and I am inclined to wear these clothes, which are modest, etc.)
Again, who are the best judges of a gymnastics competition? The answer is former gymnasts. They know a good routine from within. Those who have never mastered the sport don't have it within them, and so to them, each competitor looks almost as good as the next guy. If you are not a musician, you might judge this person to be a good guitar player, but after becoming an accomplished guitarist yourself, your judgment will change. You will know what a good guitarist is from within, and you will notice things that you did not notice previously. This is true with the arts and any other sport. The knowledge of how to dribble a basketball and throw baskets is in your muscles as a result of years of practice. You may not understand the physics of throwing a ball, but you know how to throw one nonetheless.
A particular tree grows and reproduces itself because it possesses the form of tree (the substantial form). Your idea of "tree" however does not reproduce, nor does it weigh anything. A thing, by virtue of its form, is inclined to act a certain way, that is, according to its nature (gasoline is flammable, iron resists pressure, trees grow, dogs run and bark, birds fly, humans reason, etc.). Similarly, when a person possesses a virtue - not merely the idea of justice, or chastity, but the actual disposition or habit - , he possesses the "form" within himself (the accidental form or quality). The idea "justice" for example, or "chastity", or "fortitude" are now within him, as habits that dispose the will (in the case of justice) to obey reason, that dispose the pleasures of touch or the passions of the concupiscible appetite (in the case of temperance) to readily obey reason, or that dispose the emotions of fear and daring to readily obey reason, etc. Since this person possesses these virtues or habits, he is inclined towards just, temperate, and courageous acts (for a thing always acts according to its form or nature).
To possess the art of sculpting is to be inclined to sculpt well; to possess the art of music (i.e., the violin) is to be inclined to produce beautiful music easily, etc. Similarly, a virtuous person knows the virtuous course of action from within, so to speak; for he or she is inclined to it.
Now, keep in mind that the emotions affect the intellect. Disordered passion blinds the mind. "As a person is, so does he see", said Aristotle. An unjust person who puts his own desires before the rights of others is not going to be inclined to the just course of action in a particular situation (an unjust person always thinks his rights have been violated, regardless of what the judge tries to explain to him). He is going to be inclined to that option that will bring him the most pleasure. The coward is not going to be inclined to the truly courageous course of action; he is going to be inclined to that course of action that is safest for him alone - the rest can lie down and die, for all he cares. Moreover, he sees the truly brave man as reckless. But the reckless and foolhardy man who loves to run headlong into danger unnecessarily - because it raises his adrenaline level - sees the truly brave man as a coward. So too, the sexually immoral will judge the chaste woman as a prude, and the liar judges the honest man as a "goody goody".
Recall the example of the lazy lout who is attracted to the political party who promises to take care of him, or the example of the criminal who sees his lawyer as a bad man, because the jury found him guilty of rape and murder (of which he is truly guilty). Consider the lazy student who judges as "good" a truly bad teacher who gives away marks like they are candy and teaches nothing.
In order to make prudent decisions in the concrete situations of daily life, one needs rightly ordered appetite, for only the virtuous see correctly (via connatural knowledge).
But one also needs to know universal moral principles, an understanding of how morality works, what actions are absolutely impermissible, which precepts are relative, etc. This is the science of ethics, and like any science, it has developed over time as a result of reflection and reasoning. But it can only begin in a person who has made an initial decision to love the moral good (the bonum honestum) over the delectable good (his pleasure). That is a free decision that each one of us has made (usually at a very young age) at some point in our lives (either for the good or for the self). A person who has made a decision that translates into a love of the moral good (the common good, the good of all human persons, the moral order) over his own private good, is one who will want to know more fully what is truly good, as opposed to what is only apparently good (what appears as good, but is truly evil), and so as time goes on, he will be motivated to draw out or abstract universal moral principles, such as the human goods and how we ought to relate to them, what is truly right and ordered (just), what is truly honest, temperate, brave, etc, in order to continue to grow in virtue. He will be open to the moral duty to order his passions to follow (or to line up with) what he ultimately wills, which is the moral good, and so he will have the beginnings of virtue. In other words, as a young man or even a child, he will have a degree of right appetite; he will be open to a proper upbringing, which involves parental correction, and he will be open to the correction of those who represent them (teachers, persons of authority, etc).
An immoral person will be indifferent to moral principles, truth and virtue, and often this indifference is made manifest very early on in a person's life, despite having good parents. If as a young adult he shows interest in the science of morality at all, it will be ultimately for the sake of justifying his own lifestyle choices.
The person who loves the moral good will grow in a scientific knowledge of the good - assuming he has the time to study or reflect upon these principles - , his will (the rational appetite) moves him to pursue the truth more completely, and his increasing knowledge affects his actions - he becomes more virtuous because he conforms his actions to what he discovers to be truly good, and his corresponding growth in virtue deepens his knowledge from within (connatural knowledge), which continues to drive him towards a deeper insight into the truth of what is morally right and good, and a more complete articulation of that truth.
In the midst of the particular situations of day to day life, the virtuous man is able to see (conscience) what ought to be done in the here and now, and if he cannot, experience will gradually help him to arrive at understanding because he is open to correction (he possesses docility). The immoral are not so open, which is why they tend not to learn from experience.