From ST I.29.1 ad 5:
Objection 5. Further, the separated soul is an individual substance of the rational nature; but it is not a person. Therefore person is not properly defined as ["an individual substance of a rational nature."].
Reply to Objection 5. The soul is a part of the human species; and so, although it may exist in a separate state, yet since it ever retains its nature of unibility [i.e., ability to be united], it cannot be called an individual substance, which is the hypostasis or first substance, as neither can the hand nor any other part of man; thus neither the definition nor the name of person belongs to it.
From ST I.75.2 ad 1:
Objection 1. It would seem that the human soul is not something subsistent. For that which subsists is said to be "this particular thing." Now "this particular thing" is said not of the soul, but of that which is composed of soul and body. Therefore the soul is not something subsistent....
Reply to Objection 1. "This particular thing" can be taken in two senses. Firstly, for anything subsistent; secondly, for that which subsists, and is complete in a specific nature. The former sense excludes the inherence of an accident or of a material form; the latter excludes also the imperfection of the part, so that a hand can be called "this particular thing" in the first sense, but not in the second. Therefore, as the human soul is a part of human nature, it can indeed be called "this particular thing," in the first sense, as being something subsistent; but not in the second, for in this sense, what is composed of body and soul is said to be "this particular thing."
From ST I.75.4 ad 2:
Objection 2. Further, the human soul is a substance. But it is not a universal substance. Therefore it is a particular substance. Therefore it is a "hypostasis" or a person; and it can only be a human person. Therefore the soul is man; for a human person is a man....
Reply to Objection 2. Not every particular substance is a hypostasis or a person, but that which has the complete nature of its species. Hence a hand, or a foot, is not called a hypostasis, or a person; nor, likewise, is the soul alone so called, since it is a part of the human species.
From ST II-II.85.1 ad 5:
Objection 5. Further, the soul of Peter is not Peter. If therefore the souls of the saints pray for us, so long as they are separated from their bodies, we ought not to call upon Saint Peter, but on his soul, to pray for us: yet the Church does the contrary. The saints therefore do not pray for us, at least before the resurrection....
Reply to Objection 5. It is because the saints while living merited to pray for us, that we invoke them under the names by which they were known in this life, and by which they are better known to us: and also in order to indicate our belief in the resurrection, according to the saying of Exodus 3:6, "I am the God of Abraham," etc.