Servers in Albs, Aqinas

In the Supplement to the Summa Theologica, Question 40 Article 7, we read this:

Seventh Article
Whether the Vestments of the Ministers Are Fittingly Instituted in the Church?

"I answer that, The vestments of the ministers denote the qualifications required of them for handling Divine things. And since certain things are required of all, and some are required of the higher, that are not so exacted of the lower ministers, therefore certain vestments are common to all the ministers, while some pertain to the higher ministers only. Accordingly it is becoming to all the ministers to wear the amice which covers the shoulders, thereby signifying courage in the exercise of the Divine offices to which they are deputed; and the alb, which signifies a pure life, and the girdle, which signifies restraint of the flesh.