Spiritual life has a certain conformity with the life of the body: just as other corporeal things have a certain likeness to things spiritual. Now a man attains perfection in the corporeal life in two ways: [I] in regard to his own person; [II] in regard to the whole community of the society in which he lives, for man is by nature a social animal.
With regard to himself man is perfected in the life of the body, in two ways: [IA] per se, i.e. by acquiring some vital perfection; [IB] per accidens, i.e. by the removal of hindrances to life, such as ailments, or the like. Now the life of the body is perfected per se in three ways.
[IA1] By generation whereby a man begins to be and to live: and corresponding to this in the spiritual life there is Baptism, which is a spiritual regeneration, according to Titus 3:5: "By the laver of regeneration," etc.
[IA2] By growth whereby a man is brought to perfect size and strength: and corresponding to this in the spiritual life there is Confirmation, in which the Holy Ghost is given to strengthen us. Wherefore the disciples who were already baptized were bidden thus: "Stay you in the city till you be endued with power from on high" (Luke 24:49).
[IA3] By nourishment, whereby life and strength are preserved to man; and corresponding to this in the spiritual life there is the Eucharist. Wherefore it is said (John 6:54): "Except you eat of the flesh of the Son of Man, and drink His blood, you shall not have life in you."
And this would be enough for man if he had an impassible life, both corporally and spiritually; but since man is liable at times to both corporal and spiritual infirmity, i.e. sin, hence man needs a cure from his infirmity; which cure is twofold.
[IB1] One is the healing, that restores health: and corresponding to this in the spiritual life there is Penance, according to Psalm 40:5: "Heal my soul, for I have sinned against Thee."
[IB2] The other is restoration of former vigor through appropriate diet and exercise: and corresponding to this in the spiritual life there is Extreme Unction, which removes the residue of sin, and prepares man for final glory. Wherefore it is written (James 5:15): "And if he be in sins they shall be forgiven him."
In regard to the whole community, man is perfected in two ways.
[IIA] By receiving power to rule the many and to exercise public acts: and corresponding to this in the spiritual life there is the sacrament of Order, according to the saying of Hebrews 7:27, that priests offer sacrifices not for themselves only, but also for the people.
[IIB] In regard to natural propagation. This is accomplished by Matrimony both in the corporal and in the spiritual life: since it is not only a sacrament but also an office of nature.
[St. Thomas Aquinas, Summa Theologiae 3.65.1; I have made a number of modifications to the translation. Note, incidentally, since it is often missed, that the propagation Matrimony concerns qua sacrament is propagation in the spiritual life.]
A slight variation (particularly with regard to extreme unction and with spiritual propagation being given to Order rather than Matrimony) on this scheme is found in St. Thomas's catechetical writings:
First, man needs regeneration or re-birth which is brought through the Sacrament of Baptism.... Secondly, it is necessary that man develop perfect strength, which is, as it were, a spiritual growth, and this indeed comes to him in the Sacrament of Confirmation. This is like the strengthening which the Apostles received when the Holy Ghost came upon them and confirmed them.... The third similarity is that man must be fed with spiritual food.... Fourthly, man must be healed spiritually through the Sacrament of Penance.... Lastly, one is healed both in soul and in body in the Sacrament of Extreme Unction.... Two of the Sacraments, Orders and Matrimony, are instituted for the common good of the Church. Through the Sacrament of Orders the Church is ruled and is spiritually multiplied; and through Matrimony it is increased physically in numbers.
It is worth comparing this to the similar scheme (same with also a slight, and different, modification for extreme unction) of St. Robert Bellarmine:
Now, the reason why there are seven is that God customarily proceeds, in the way in which spiritual life is given, in an incorporeal manner. In as much as He considers the corporeal life: 1) it is necessary to be born; 2) to grow; 3) to be nourished; 4) whenever one must fight, he must be armed; 6) it is necessary that there be some head that rules all men after they are born and increased; 7) that there would be some to whom the duty to propagate the human race would fall, otherwise, if others were not born to succeed the dead the human race would forthwith go out of existence.In the same way, God also constituted this arrangement in the spiritual life. 1) It is necessary for us to be born in the grace of God through Baptism; 2) Confirmation makes it so that grace will grow and be fortified; 3) the Most Holy Eucharist is given so that grace might be nourished and sustained; 4) that whenever the medicine of Penance is received, the grace lost to the soul will be recovered; 5) that when a man is at the point of death he arms itself against the infernal enemy who attacks us more at that time than in any other time, which is done with Extreme Unction; 6) that there would be someone in the Church that rules and governs us in the spiritual life, which is done by one in Orders; 7) that there would be some in the Church who look to the spiritual propagation of the human race, whereby the number of faithful could be increased in this way, which duty is carried out in the Sacrament of Matrimony.[St. Robert Bellarmine, Doctrina Christiana: The Timeless Catechism of St. Robert Bellarmine, Grant, tr. Mediatrix Press (pp. 153-154).]
Being the preferred scheme of Aquinas, the most widely read Doctor of the Church, and the scheme used by Bellarmine in his catechism, which for a long time was the most widely read Catholic catechism (and influenced a number of other catechisms), this scheme based on analogy with natural life is far and away the most common one. But, as we shall see the next few days, there are others based on analogy with medicine, with moral life, and with equipping an army.
With regard to the slight variations, all of them seem to work as they are; my own view is that Aquinas's catechesis on the sacraments has the best account of Unction (the Summa seems to give us only an indirect analogy, and Bellarmine seems to give us a secondary feature), but both Bellarmine and the Summa give a better account of Matrimony than it does (because spiritual propagation should in fact be assigned to Matrimony rather than Order). The reason is that sacramental theology in Church documents has stabilized in primarily thinking of anointing the sick as based on overflow of grace from spirit to body, while there has been an increasing recognition of the importance of Matrimony as the sacrament that forms the 'domestic church'; Order, on the contrary, is more concerned with maintaining and protecting the integrity of the whole sacramental economy.