The following passage is taken from Garrigou-Lagrange's Three Ages of the Interior Life. This is the first section of a chapter on the passions. I will post the other sections in subsequent posts.
There can be no interior life without a struggle against self in order to regulate and discipline the passions, to cause the light of right reason and even that of infused faith and of Christian prudence to descend into these movements of the sensible appetites. There is far more than we think in the expression, to discipline one's sensible appetite; it should receive discipline like a docile pupil who is being trained. Consequently it is fitting that we speak of the passions. To proceed in an orderly fashion, we must consider them from the psychological, the moral, and the essentially ascetical point of view. We shall follow the teaching of St. Thomas.(1)
THE PASSIONS FROM THE PSYCHOLOGICAL POINT OF VIEW
St. Thomas, who follows Aristotle and St. John Damascene, defines passion thus: "A movement of the sensitive appetite when we imagine good or evil. . . . A passion is properly to be found where there is corporeal transmutation." (2)
When we say that it is a movement of the sensible appetite, common to man and animal, a distinction is made between passion and a movement of the spiritual will, called the rational appetite. Neither must the movement of the sensible appetite be confused with corporeal movements: for example, with the beating of the heart that follows it. These movements of the sensitive appetite which are the passions manifestly exist in the animal: for example, when it desires its food, and in it passion is now under a mild form, as in the dove or the lamb, now under a violent form, as in the wolf, the tiger, or the lion.
Following Aristotle, St. Thomas distinguishes and classifies the different passions in a remarkable manner. He distinguishes first of all the concupiscible appetite, which inclines one to seek for sensible and delectable good and to flee injurious evil, and the irascible appetite, which inclines one to resist obstacles and, in spite of them, to obtain a difficult good. There are animals and men dominated by the irascible appetite, others dominated by the concupiscible.
In the concupiscible appetite, in regard to sensible good which attracts, three passions are distinguished: the love of this sensible good, whether it is present or absent; the desire of this good, if it is absent; the joy, if it is present. These movements of the sensible appetite are seen in the animal to which food is brought or from which it is removed.
On the contrary, in reference to evil to be avoided, we distinguish in the concupiscible, hatred, aversion, and sadness. Thus the lamb instinctively flees from the wolf.
In the irascible appetite, in reference to the good difficult to obtain (bonum arduum), there are the two passions of hope and of despair or dejection, according as this good appears obtainable or unobtainable. And in this same appetite, with regard to injurious evil to be repulsed, there is audacity and fear, according as this evil is easy or difficult to repulse, and also anger, if it is a question of a present evil to be surmounted or an insult to be avenged.
In the spiritual will there are analogous movements of love, desire, joy, hope, and so on, but these are of an immaterial order, whereas the passion is always accompanied by a movement of the organism, because of the fact that the sensible appetite is united to an organ.
Among all the passions, the first of all, presupposed by all the others, is sensible love: for example, in the animal, love of the food that it needs. From this love are born desire, joy, hope, audacity, or hatred of what is contrary, aversion, sadness, despair, fear, anger.(3)
From what we have said, it is evident that passion, as it has been defined, is not always lively, vehement, and dominant. However, many modern authors apply the term "passion" to a particularly intense movement of the sensible appetite and reserve "emotion" to others that are less strong.
1. Cfr. Ia IIae, q.22-28.
2. Ibid., q.22, a.3.
3. Cf. Bossuet, De la connaissance de Dieu et de soi-meme, chap. I, § 6.