I answer that, As is clear from what has been stated above (Q. 22, A. 3; Q. 103, A. 6), Divine Providence produces effects through mediate causes. We can therefore consider the ordering of the effects in two ways. Firstly, as being in God Himself: and thus the ordering of the effects is called Providence. But if we consider this ordering as being in the mediate causes ordered by God to the production of certain effects, thus it has the nature of fate. This is what Boethius says (De Consol. iv): "Fate is worked out when Divine Providence is served by certain spirits; whether by the soul, or by all nature itself which obeys Him, whether by the heavenly movements of the stars, whether by the angelic power, or by the ingenuity of the demons, whether by some of these, or by all, the chain of fate is forged." Of each of these things we have spoken above (A. 1; Q. 104, A. 2; Q. 110, A. 1; Q. 113; Q. 114). It is therefore manifest that fate is in the created causes themselves, as ordered by God to the production of their effects.
Reply Obj. 1: The ordering itself of second causes, which Augustine (De Civ. Dei v, 8) calls the "series of causes," has not the nature of fate, except as dependent on God. Wherefore the Divine power or will can be called fate, as being the cause of fate. But essentially fate is the very disposition or "series," i.e. order, of second causes.
Reply Obj. 2: Fate has the nature of a cause, just as much as the second causes themselves, the ordering of which is called fate.
Reply Obj. 3: Fate is called a disposition, not that disposition which is a species of quality, but in the sense in which it signifies order, which is not a substance, but a relation. And if this order be considered in relation to its principle, it is one; and thus fate is one. But if it be considered in relation to its effects, or to the mediate causes, this fate is multiple. In this sense the poet wrote: "Thy fate draws thee." _______________________
THIRD ARTICLE [I, Q. 116, Art. 3]
Whether Fate Is Unchangeable?
Objection 1: It seems that fate is not unchangeable. For Boethius says (De Consol. iv): "As reasoning is to the intellect, as the begotten is to that which is, as time to eternity, as the circle to its centre; so is the fickle chain of fate to the unwavering simplicity of Providence."
Obj. 2: Further, the Philosopher says (Topic. ii, 7): "If we be moved, what is in us is moved." But fate is a "disposition inherent to changeable things," as Boethius says (De Consol. iv). Therefore fate is changeable.
Obj. 3: Further, if fate is unchangeable, what is subject to fate happens unchangeably and of necessity. But things ascribed to fate seem principally to be contingencies. Therefore there would be no contingencies in the world, but all things would happen of necessity.
On the contrary, Boethius says (De Consol. iv) that fate is an unchangeable disposition.