In Reformed circles, there has been a controversy regarding the nature of saving faith — whether it be dipartite (two parts) or tripartite (three parts). Both sides accept cognitio and assentia in the definition of faith. Those who follow Van Til (the majority) tend to add fiducia as a third element of faith, while Gordon Clark denied fiducia as being a proper element, not disagreeing with the concept but subsuming it under the element of assentia, noting that adding it as an element is tautological, since the root of Fiducia is Fide, which is basically Latin for "faith".
While the debate would properly not go away any time soon, no matter how much Van Tillians would like that to be, where does Witsius land on this issue?
According to his The Economy of the Covenants, Witsius does not put trust (Fiducia) as one of the elements of faith. Following on knowledge (cognitio) (III.7.8) and assent (assentia) (III.7.11), the next "acts of faith" are "love of the truth" (III.7.17), then followed by "a hunger and thirst after Christ" (III.7.18), "a receiving of Christ the Lord for justification, sanctification, and so for complete salvation" (III.7.19), "recline and stay itself upon him [Christ]" (III.7.21), "surrender to Christ" (III.7.23), and to "conclude, that Christ with all his saving benefits are his" (III.7.24). A careful reading of Witsius however show us these other acts of faith are indeed ACTS of faith, not elements of faith. Starting from the third act of the "love of the truth", we read thus:
It is indeed true, that love, strictly speaking, is distinguished from faith; yet the act of both virtues, or graces, are so interwoven with one another, that we can neither explain nor exercise faith without some acts of love interfering; ... (III.7.17. Vol. 1, p. 381)
These other acts of faith therefore are rather the fruits of faith, not the elements of faith strictly speaking.
However, between discussing the second and third acts of faith, it can be seen that Witsius qualifies the assentia of saving faith by linking them with the idea of it having substance, using the Greek word ύποστασις (of which the nuance cannot be fully expressed when translated) to modify the assent that is required. In Witsius' words:
XIII. The term, ύποστασις hypostasis substance, is also very emphatical, which the apostle makes use of when he speaks of faith, Heb. xi. 1. Nor have the Latins any word that can fully express all its force and significancy. 1st Ύποστασις hypostasis denotes the existence, or, as one of the ancients has said, the extantia, the standing up of a thing; in which sense philosophers say that a thing that really is has an ύποστασις, that is, real existence, and is not the fiction of our own mind. An indeed faith makes the thing hoped for, though not actually existing, to have, notwithstanding, an existence in the believer's mind, who so firmly assents to the promises of God, as if the thing promised was already present with him. ... And [John] Calvin's interpretation looks this way; faith, says he, is hypostasis, that is, a prop or possession on which we fix our feet. ... And indeed there is something in faith that can with intrepidity sustain all the assaults of temptations, and not suffer it to be moved from an assent to a truth once known. Now it we join all this together, we may assert, that faith is so firm an assent to divine truth, as to set things future before us as if they were present, and that it is a proper to the soul on which it fixes its foot without yielding to any assault whatever. (III.7.18. Vol. 1, pp. 378-379. Emphases original)
From what can be seen, Witsius does not seem to fit any of the two categories. Maybe we should change the term "fiducia" (trust) to "hypostasis" (substanced confidence?)??