Objection 1. It would seem that initial fear differs substantially from filial fear. For filial fear is caused by love. Now initial fear is the beginning of love, according to Sirach 25:16, "The fear of God is the beginning of love." Therefore initial fear is distinct from filial fear.
Objection 2. Further, initial fear dreads punishment, which is the object of servile fear, so that initial and servile fear would seem to be the same.
But servile fear is distinct from filial fear. Therefore initial fear also is substantially distinct from initial fear.
Objection 3. Further, a mean differs in the same ratio from both the extremes. Now initial fear is the mean between servile and filial fear. Therefore it differs from both filial and servile fear.
On the contrary, Perfect and imperfect do not diversify the substance of a thing. Now initial and filial fear differ in respect of perfection and imperfection of charity, as Augustine states (In prim. canon. Joan. Tract. ix). Therefore initial fear does not differ substantially from filial fear.
I answer that, Initial fear is so called because it is a beginning [initium]. Since, however, both servile and filial fear are, in some way, the beginning of wisdom, each may be called in some way, initial.
It is not in this sense, however, that we are to understand initial fear in so far as it is distinct from servile and filial fear, but in the sense according to which it belongs to the state of beginners, in whom there is a beginning of filial fear resulting from a beginning of charity, although they do not possess the perfection of filial fear, because they have not yet attained to the perfection of charity. Consequently initial fear stands in the same relation to filial fear as imperfect to perfect charity. Now perfect and imperfect charity differ, not as to essence but as to state. Therefore we must conclude that initial fear, as we understand it here, does not differ essentially from filial fear.
Reply to Objection 1. The fear which is a beginning of love is servile fear, which is the herald of charity, just as the bristle introduces the thread, as Augustine states (Tract. ix in Ep. i Joan.). Or else, if it be referred to initial fear, this is said to be the beginning of love, not absolutely, but relatively to the state of perfect charity.
Reply to Objection 2. Initial fear does not dread punishment as its proper object, but as having something of servile fear connected with it: for this servile fear, as to its substance, remains indeed, with charity, its servility being cast aside; whereas its act remains with imperfect charity in the man who is moved to perform good actions not only through love of justice, but also through fear of punishment, though this same act ceases in the man who has perfect charity, which "casteth out fear," according to 1 John 4:18.
Reply to Objection 3. Initial fear is a mean between servile and filial fear, not as between two things of the same genus, but as the imperfect is a mean between a perfect being and a non-being, as stated in Metaph. ii, for it is the same substantially as the perfect being, while it differs altogether from non-being.
Source: Summa Theologica (New Advent)