One of the many puzzles currently of interest to scholars studying Malebranche is the problem of the efficacy of ideas. Jolley summarizes this in a tidy way:
Perhaps the most puzzling of Malebranche's claims bout ideas is that they have causal properties; ideas have the power to cause perceptions in finite minds. But if ideas are third realm entities, it is hard to see how they can have causal properties of any sort. It is true that ideas are located in God, and God is traditionally thought of as causally active; but if God is causally active, it is surely not qua region of ideas.
He highlights a related problem when discussing the problem of the ontological status of ideas:
For if ideas are identified with the substance of God, then the infinite, uncreated res cogitans of Descartes' metaphysics is converted into an abstract entity or perhaps the realm of abstract ideas....[A]nd apart from the problem of philosophical inconsistency, it also generates theological difficulties; it is fundamentally at odds with the orthodox Christian conception of God as a personal being who watches over us and cares for us.
We have, then, two problems that derive from thinking of Malebranche's ideas as 'third realm' or abstract entities. They both gain their plausibility from the fact that ideas as Malebranche understands them certainly do some of the things that 'third realm' or abstract entities are supposed to do. In what follows, however, I will suggest that a different view of the problem arises when we take seriously Malebranche's insistence that ideas are divine ideas in the natural-theological sense. In this context ideas are an artisan's knowledge of things that can be produced, and they are intimately involved in the production of those things. His account of ideas should be taken to have more in common with this than with what we ordinarily think of as 'third realm' or abstract entities.
Divine Ideas as Practical Knowledge
It is immediately clear that the key feature of Malebranche's theory of ideas is the identification of our ideas with divine ideas; this identification is just the thesis that we see all things in God. It is also clear, however, that Malebranche already has a notion of what a divine idea is. This notion is laid out most clearly in some of Malebranche's later works. In the 1696 Preface to the Dialogues on Metaphysics and on Religion, he appeals to a number of texts by Augustine and Aquinas in order to support his account of ideas. For instance, Malebranche links his own accoutn of ideas to principles he finds in Augustine's LXXXIII Questions (q. xlvi): ideas are importat because "without them one cannot be wise"; they are eternal and immutable; they are the exemplars or archetypes of creatures; they are not separate from the divine essence (as a Platonic world of ideas) but are in God; the multiplicity of ideas does not contradict the doctrine of divine simplicity (OC 12:11). He appeals to similar principles occurring in Aquinas's discussions. By 1696, at least, Malebranche saw himself to be defending an account of ideas that merges with a traditional theology of divine ideas, such as we find in Aquinas's development of Augustine. In light of htis, I suggest that it might be useful to examine Aquinas's theology of divine ideas, as a reasonably accessible account of the traditional understanding of divine ideas, and one with which Malebranche himself was familiar, if we want to shed light on Malebranche's claims that ideas are efficacious.
Aquinas has a number of passages in which he appears to attribute causal efficacy to God' knowledge. For instance, in De Veritate (q. 2, art. 14), discussing whether God's knowledge is the cause of created things, he insists that it is. In a later discussion, he explicitly says that ideas in the divine mind create or produce things (DV q. 3, art. 1 ad 5). We find similar claims in the Summa Theologiae:
It must be said that God's knowledge is the cause of things. For the knowledge of God is related to all created things as the knowledge of the artificer is to the artifact. (ST 1.14.8)
There is some uncertainty about how these claims should be interpreted. Elders has argued that we should take them at face value. On this interpretation, God's knowledge is "creative knowledge." As Elders summarizes the position, God's knowledge is causal knowledge so that God knows things because he makes them. Sump has argued for a different interpretation. As she characterizes Elders's interpretation, it involves one of two claims:
(1) God knows everything that he knows in virtue of his knowledge's being the efficient cause of what he knows;
(2) God knows everything both temporal and actual that he knows in virtue of his konwledge's being the efficient cause of this part of what he knows. 
(1) is much stronger than (2). However, as Stump points out, even (2) is too strong; it cannot explain what Aquinas says about God's knowledge of evil, for instance. She suggests that the plausibility for these stronger claims rests on two further assumptions:
(A) The causation which God's knowledge has is efficient causation;
(B) What is effected by the causation of the divine cognition includs all actions, events, and states of affairs in the world.
(B) is irrelevant to our purposes; we simply want to know whether any of God's knowledge is (in any reasonably straightforward sense) efficacious.
Stump's claim is that when Aquinas speaks of God's knowledge as causative, he means that God's knowledge is a formal cause, not an efficient cause. To this end she emphasizes passages in which Aquinas identifies ideas as forms and exemplars (we have already noted some of the passages in which Aquinas does so). From this she concludes that ideas must be formal causes: the pattern a builder has in mind as he begins to build a house is the formal cause of the house, not an efficient cause.
It is clearly true that ideas are exemplars and forms. What is less clear is whether Stump's argument shows that God's knowledge is not effectively causative. An exemplar cause is not an intrinsic cause but an extrinsic cause, since the divine idea of a body is not the constitutive form of that body, but the pattern on which it is made. As Aquinas says, they are formae aliarum, praeter ipsas res existentes: the forms of all things, existing apart from the things themselves. This last qualification, separate existence, should not be ignored. When we identify divine ideas as forms and exemplars, we need to ask a further question: How are they related to the (separate) things of which they are exemplars? When we consider this question, we find reason to say that Thomas's view makes God's knowledge in some sense efficacious. One place where I think this becomes very clear is the discussion of divine ideas in De Veritate.
At various times in De Veritate, Aquinas appears to attribute to divine ideas the characteristics of formal, final, and efficient causes. As we have noted, he is very clear that divine ideas are formal causes (DV q. 3, art. 1). In the same place, he says that a divine idea "has in a certain way the nature of an end," giving this as a reason for rejecting the Platonic claim that ideas are separate from God. It is also in this same article that he makes the claim, noted above, that ideas in the divine mind create and produce things (DV q. 3, art. 1 ad 5). While there is no doubt that ideas are exemplar causes, it seems plausible to suggest that Aquinas thinks divine ideas are 'in a certain way' efficient causes, just as we have seen that he thinks they are 'in a certain way' final causes. This argument presupposes a particular conception of practical knowledge, which it will be worthwhile to examine.
After noting that practical knowledge is called practical because it is directed to a work (of some kind), Aquinas distinguishes this form of knowledge into two kinds. In actually practical knowledge, the form that the artisan has in mind is actually directed to the work; in habitually and virtually practical knowledge, the form, while able to be directed to the work, is not actually so directed. God has both kinds of practical knowledge:
Since his knowledge causes things, He knows some things by ordaining by a decree of His will that they be at a certain time, and of these he has actual practical knowledge. Moreover, He knows other things He does not at any time intend to make, for He knows those things which are not, have not been, and never will be. Of these things He has actual knowledge; not actually practical, however, but merely virtually practical. (DV q. 3, art. 3)
In light of this distinction between types of practical knowledge, it can be seen that one possible argument against the view that God's knowledge is efficacious fails. One of Stump's arguments that the common interpretation fails is that it cannot account for God's knowledge of 'things that are not'. This is true if we understand the common interpretation to be claiming that God knows only what he actually causes. Stump is entirely correct that this cannot be right; Aquinas himself denies that God's knowledge of evil is causative (DV q. 3, art. 3), and her argument is certainly a salutary warning about the dangers of incautious claims. On a less restrictive account like the one suggested here, God knows the exercise of His causal power as part of a more general knowledge of His causal power itself. As Aquinas notes, we have to allow for practical knowledge of things that might exist even though they do not, e.g., a builder's practical knowledge of a house he could make but never does. God's knowledge of creatures that do not exist but could is necessarily a form of causal knowledge; the only way God can know anything about such creatures is to know what He could make. When He actually makes creatures, this adds an additional kind of causal knowledge beyond the more general background knowledge.
At this point it may be useful to consider a possible objection. Aquinas is very clear that knowledge as such is not efficacious. Two passages in particular are worth noting:
Knowledge, inasmuch as it is knowledge, is not called an active cause...form is not a principle of acting save by mediation of a power...from knowledge never proceeds an effect save by mediation of a will...the things known by God proceed from His knowledge by way of His will and by way of secondary causes. (DV q. 2, art. 14)
A natural form, inasmuch as it is the form dwelling in that to which it gives being, is not called a principle of action, but only with regard to its having an inclination to the effect. And similarly the intelligible form is not called a principle of action with regard to its being in the intellect, unless there is adjoined to it an inclination to the effect, which is by the will. (ST 1.14.8)
Upon reading passages like this, someone might say that, whatever Aquinas's actual view, he cannot consistently regard God's knowledge as efficacious in itself. If an effect never arises from knowledge save through the mediation of the will, the really effective cause must be the divine will.
An answer to this objection is suggested by Aquinas's example of the artisan and the artifact. Imagine yourself to be a painter (artisan) painting a canvas (the painted canvas being the artifact). This activity is intentional; it is an expression of will. However, it is not an expression of will alone. Even the mere intent to paint involves not merely will but an intellect that knows and believes things relevant to painting, e.g., what painting is and what sort of things might be painted. Without this the will could not be efficacious. An effect must be linked to its cause in some way in order to be identifiable as the effect of this cause. Something about the painter must make it possible to say that the painter is efficacious, not merely simpliciter, but also qua painter. In other words, in an intentional and deliberate act of painting, linking the painting (effect) to the painter's will (cause) requires that we be able to postulate some content to the will that makes it a will to paint. It is difficult to see what else could give the will this content except the intellect. If it is the intellect that gives the content, the will is not efficacious on its own; rather, it is only efficacious insofar as the intellect has given it an object.
That Aquinas has something like this account in mind when discussing causative knowledge is suggested by what Aquinas immediately goes on to say after the second passage quoted above:
For since the intelligible form is related to opposites (since the same knowledge is of opposites) it would not produce a determinate effect unless it were determined to one by appetite, as is said in Metaph. IX. And it is manifest that God causes things by His intellect, since His being is His understanding. Hence it is necessary that His knowledge be the cause of things insofar as it has the will conjoined to it. Hence God's knowledge, insofar as it is cause of things, is customarily called the knowledge of approbation. (ST 1.14.8)
In other words, the fact that knowledge is not efficacious on its own is not an impediment to its being considered a cause insofar as it is conjoined with the will. This is not surprising. For any particular effect, the will is not efficacious on its own either, since it does not operate blindly; it is a cause insofar as it is conjoined with the intellect. The will is not sufficient for producing a given effect unless the intellect presents it with that effect as an object; it is only a necessary condition for the actual production of an effect. Therefore, an effect is traced back not to one or the other but to the two together, and each can be considered a causal principle. Each is, in fact, a causally necessary condition. This is particularly true in God's case, since, as Aquinas notes in the above passage, divine simplicity make sit even harder to say that only the divine will is efficacious. God makes everything He makes by means of the divine will; but it is also true that He makes all things by means of His intellect.
In the next section I will suggest that Malebranche has an account of ideas in which ideas are a kind of practical knowledge in something like the sense I have suggested in Aquinas's case. Malebranche's account, however, is complicated by his tendency to run together discussion of divine Reason (understood hypostatically as the divine Word, the second person of the Trinity) and discussion of divine ideas. It is worth a moment of our time, then, to pause and look very briefly at what the relation between idea and Word is in Aquinas's account.
Aquinas accepts the statement from the prologue of the Gospel of John that all things God made were made through the Word (Logos) of God, and understands "the Word" in the standard way to refer to the second person of the Trinity. The claim that all things made were made through the Word allows for a connection between the theory of divine ideas and the theory of the Divine Word, since, as Thomas describes it, "in 'Word' is implied the operative idea of what God makes" (ST 1.34.3). These operative ideas of what God makes are instances of what he elsewhere calls the likenesses of things in the Word:
Just as the likenesses of things in the Word are the causes of the existing of things, so also they are the causes of knowing what is, insofar as they are impressed into intellectual minds, thus causing them to be able to know things. Hence, just as these likenesses are called life because they are principles of existing, so are they also called light because they are principles of knowing. (DV q. 4, art.8 ad 4)
These likenesses int he Word, in other words, cause any given created thing both to exist and to be known. Aquinas denies, of course, that in our present life the divine ideas are direct objects of knowledge; but he insists that we know all things through them in a way analogous to the way we see things by the sun (ST 1.84.5). Going further into detail with this would require delving into his theory of participation, which I will forego here. However, this does seem clear enough: divine ideas in the divine Word are both "principles of existing" and "principles of knowing".
Despite the connections between the theory of ideas and the theory of the Word, Aquinas thinks we need to draw a sharp distinction between the two. In De Veritate Aquinas considers two arguments that conflate divine ideas with the Word. To the first he replies:
A word differs from an idea, of 'idea' names an exemplary cause absolutely, but the 'Word' of a creature in God names an exemplary form drawn from something else, and thus 'idea' in God pertains to the essence, but 'Word', to a person. (DV q. 4, art. 4 ad 4)
Aquinas's chief concern here is to prevent a conflation of natural theology and revealed theology. Ideas, a matter of natural theology, simply identify the forms in God's mind that He contemplates in His creation of the world. Since God is simple, these forms are just the divine essence considered in different lights. However, a Christian, reflecting on the first few verses of the Gospel of John, will discover that God makes things through the Logos, which is to say, the Word or Reason. Thomas wants to shut down the temptation to conflate ideas and the Word; he recognizes that the terms 'idea' and 'Word' both suggest exemplary causality, but notes that they are used in radically different ways. The ideas are the divine essence; but the Word is a divine person possessing that essence.
In the response that follows, Thomas identifies another difference between an idea and the Word: ideas express creatures directly, and are therefore many; the Word, expressing creatures only as a consequence of expressing God, is one. The same point is found in the Summa Theologiae:
The noun 'idea' is principally imposed to signify relation to creatures; and therefore it is applied plurally to God and is not said personally. But the noun 'word' is principally imposed to signify relation to the speaker, and thereafter to creatures inasmuch as God by understanding Himself understands all creatures. For this reason there is only one Word in God, and it is personal. (ST 1.34.3 ad 4)
We shall see that this distinction between idea and Word is much stronger than the one found in Malebranche's account. On that account, while ideas are many and the Word is one, this is because each idea is the Word itself considered insofar as it is able to be imitated in one particular way. This shift in emphasis has the result of strengthening Malebranche's claim that ideas are efficacious.
 Discussion of this problem in modern scholarship begins with Robinet, Système et existence dans l'oeuvre de Malebranche, 259-284. See Alquié (Le Cartésianisme de Malebranche, 208-212) and Gueroult (Malebranche, vol. 1, 110-111) for other influential discussions.
 Jolley, The Light of the Soul, 76 (cp. 111). See also Jolley, "Berkeley, Malebranche, and Vision in God," Journal of the History of Philosophy (1996), 542, and Pyle, Malebranche, 88.
 Jolley, The Light of the Soul, 79.
 One important issue involved in Malebranche's identification of ideas with divine ideas that this paper will not discuss at all is the thorny issue of whether he is (contrary to his intention) committed to saying that we see the divine essence. Connell, The Vision in God, 243-253 (see also 308-314) has a good discussion of this and related issues arising from Malebranche's melding of Cartesianism, Augustinianism, and Scholasticism.
 See also LO 229, 319, 625.
 Elders, The Philosophical Theology of St. Thomas Aquinas, 234.
 Elders, The Philosophical Theology of St. Thomas Aquinas, 238.
 Stump, Aquinas, 179. I am not convinced that Elders is committed to anything as strong as (1). Stump's description of Elders's position makes it sound as if Elders were giving a general account of divine knowledge in saying that God's knowledge is causal. Elders, however, argues that God's knowledge is causative in a section where he is explicitly discussing God's knowledge of things other than Himself.
 Stump, Aquinas, 179.
 Stump, Aquinas, 180.
 This is further suggested, at least as a possibility for interpretation, both by the use of the passage from Aristotle's Physics found in the sed contra for this article ("the three causes coincide in one, namely, the efficient, final, and formal causes") and the account of what a divine idea is in the main body: "It must be a form which something imitates due to the intention of an agent who predetermines the end himself."
 Aaron Martin has a good discussion of this in "Reckoning with Ross: Possibles, Divine Ideas, and Virtual Practical Knowledge," Proceedings of the American Catholic Philosophical Association 78 (2004) 193-208.
 For a discussion of this, see Boland, Ideas in God According to Saint Thomas Aquinas, 252.
 This line of reasoning is similar to that given in one of the sed contra arguments raised when Aquinas considers whether God knows things that are not:
A cause does not depend on its effect. But the idea is the cause of the being of the thing. Therefore, it does not depend on the being of the thing in any way. Therefore, there can be ideas of those things that are not, have not been, and never will be. (DV q. 3, art. 6 s.c. 2)
Thus, the causative character of the idea does not depend on its actually making the effect exist. As Aquinas says in the responses to the objections, "even though God may never will to produce into being in any way those things whose ideas He possesses, He wills to be able to produce them and to have the knowledge for producing them" (DV q. 3, art. 6 ad 3) and "those ideas re not ordered by divine knowledge to the production of something made according to them, but rather to this, that something can be produced according to them" (DV q. 3, art. 6 ad 4).
 Cf. Elders, The Philosophical Theology of St. Thomas Aquinas, 234.
 See also Aquinas's discussion, in ST 1.88.3, of whether God is the first object we know.