What exactly is a soul?
The glossary at the back of the U.S. version of the Catechism of the Catholic Church defines “soul” as follows:
The spiritual principle of human beings. The soul is the subject of human consciousness and freedom; soul and body together form one unique human nature. Each human soul is individual and immortal, immediately created by God. The soul does not die with the body, from which it is separated by death, and with which it will be reunited in the final resurrection.
The human person, created in the image of God, is a being at once corporeal and spiritual. The biblical account expresses this reality in symbolic language when it affirms that “then the Lord God formed man of dust from the ground, and breathed into his nostrils the breath of life; and man became a living being.” Man, whole and entire, is therefore willed by God. In Sacred Scripture the term “soul” often refers to human life or the entire human person. But “soul” also refers to the innermost aspect of man, that which is of greatest value in him, that by which he is most especially in God’s image: “Soul” signifies the spiritual principle in man. The human body shares in the dignity of “the image of God”: it is a human body precisely because it is animated by a spiritual soul, and it is the whole human person that is intended to become, in the body of Christ, a temple of the Spirit. Man, though made of body and soul, is a unity.