Objection 1. It would seem that the definition of person given by Boethius (De Duab. Nat.) is insufficient--that is, "a person is an individual substance of a rational nature." For nothing singular can be subject to definition. But "person" signifies something singular. Therefore person is improperly defined.
Objection 2. Further, substance as placed above in the definition of person, is either first substance, or second substance. If it is the former, the word "individual" is superfluous, because first substance is individual substance; if it stands for second substance, the word "individual" is false, for there is contradiction of terms; since second substances are the "genera" or "species." Therefore this definition is incorrect.
Objection 3. Further, an intentional term must not be included in the definition of a thing. For to define a man as "a species of animal" would not be a correct definition; since man is the name of a thing, and "species" is a name of an intention. Therefore, since person is the name of a thing (for it signifies a substance of a rational nature), the word "individual" which is an intentional name comes improperly into the definition.
Objection 4. Further, "Nature is the principle of motion and rest, in those things in which it is essentially, and not accidentally," as Aristotle says (Phys. ii). But person exists in things immovable, as in God, and in the angels. Therefore the word "nature" ought not to enter into the definition of person, but the word should rather be "essence."
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 above.
I answer that, Although the universal and particular exist in every genus, nevertheless, in a certain special way, the individual belongs to the genus of substance. For substance is individualized by itself; whereas the accidents are individualized by the subject, which is the substance; since this particular whiteness is called "this," because it exists in this particular subject. And so it is reasonable that the individuals of the genus substance should have a special name of their own; for they are called "hypostases," or first substances.
Further still, in a more special and perfect way, the particular and the individual are found in the rational substances which have dominion over their own actions; and which are not only made to act, like others; but which can act of themselves; for actions belong to singulars. Therefore also the individuals of the rational nature have a special name even among other substances; and this name is "person."
Thus the term "individual substance" is placed in the definition of person, as signifying the singular in the genus of substance; and the term "rational nature" is added, as signifying the singular in rational substances.
Reply to Objection 1. Although this or that singular may not be definable, yet what belongs to the general idea of singularity can be defined; and so the Philosopher (De Praedic., cap. De substantia) gives a definition of first substance; and in this way Boethius defines person.
Reply to Objection 2. In the opinion of some, the term "substance" in the definition of person stands for first substance, which is the hypostasis; nor is the term "individual" superfluously added, forasmuch as by the name of hypostasis or first substance the idea of universality and of part is excluded. For we do not say that man in general is an hypostasis, nor that the hand is since it is only a part. But where "individual" is added, the idea of assumptibility is excluded from person; for the human nature in Christ is not a person, since it is assumed by a greater--that is, by the Word of God. It is, however, better to say that substance is here taken in a general sense, as divided into first and second, and when "individual" is added, it is restricted to first substance.
Reply to Objection 3. Substantial differences being unknown to us, or at least unnamed by us, it is sometimes necessary to use accidental differences in the place of substantial; as, for example, we may say that fire is a simple, hot, and dry body: for proper accidents are the effects of substantial forms, and make them known. Likewise, terms expressive of intention can be used in defining realities if used to signify things which are unnamed. And so the term "individual" is placed in the definition of person to signify the mode of subsistence which belongs to particular substances.
Reply to Objection 4. According to the Philosopher (Metaph. v, 5), the word "nature" was first used to signify the generation of living things, which is called nativity. And because this kind of generation comes from an intrinsic principle, this term is extended to signify the intrinsic principle of any kind of movement. In this sense he defines "nature" (Phys. ii, 3). And since this kind of principle is either formal or material, both matter and form are commonly called nature. And as the essence of anything is completed by the form; so the essence of anything, signified by the definition, is commonly called nature. And here nature is taken in that sense. Hence Boethius says (De Duab. Nat.) that, "nature is the specific difference giving its form to each thing," for the specific difference completes the definition, and is derived from the special form of a thing. So in the definition of "person," which means the singular in a determined "genus," it is more correct to use the term "nature" than "essence," because the latter is taken from being, which is most common.
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, 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.
Source: Summa Theologica (New Advent)