There has long been a dispute in Catholic sacramentology over whether the efficacy of the sacraments is 'physical' or 'moral'. The terms don't mean here what they usually mean; 'physical' means here only that the causality is by some quality, capacity, capability, impulse, or force provided to the sacrament (or sacramental effect, depending on the exact version) itself. 'Moral' means that the sacrament causes specifically by being a reason for God to cause an effect. In these senses, grace itself always works by physical causality, while petitionary prayer always works by moral causality, and so the question is whether the sacraments themselves are more like the former or the latter. Everybody recognizes that God acts in the sacraments as a 'physical' cause.
One of the things that has plagued this discussion is rampant equivocation. For instance, the Sacrae Theologiae Summae VI (which is all-in on moral causality) gives the following argument (tr. 1 ch. 3. art. 4 th. 10 sect. 89):
A sacrament is a moral composite, consisting of physical parts somewhat separated in time among themselves. Therefore the physical power of acting cannot be attributed to the composite as such, because it is not a physical entity. Therefore, to what should it be attributed? to the matter? to the form? to which part of the form? (p. 74)
But this equivocates on both 'moral' and 'physical'. Sacraments, which are artifacts, are indeed moral composites -- that means that they are put together by will rather than naturally being composed the way they are -- but this is irrelevant to the question of whether they are moral causes, which is about whether they cause grace instrumentally by being provided, by God, a capability qua instrument through which he acts, or whether they cause grace instrumentally by providing God a reason to cause grace because of them. The claim that a physical power of acting cannot be attributed to a moral composite is simply wrong, when we use the term 'physical' in the correct way; if this were true, a hammer (which is a moral composite) could not be given the force to hit a nail, and a series of dominos (which is also a moral composite, and one with physical parts somewhat separated in time among themselves) could not be set to induce a falling motion in each other that achieves some result at the end.
Since none of the sacraments seem to work exactly the same way, it is perhaps not surprising that which sacrament we are considering changes considerably the arguments we have to use to argue this question. Matrimony and Reconciliation both have features that put the moral causality view on very strong ground -- Matrimony is a covenant and Reconciliation a tribunal, which are both things we at least sometimes already associate with moral causality (in the sacramental sense, not necessarily other senses): contracts and courts often effect things by providing agents reasons for doing something. On the other hand, the Church Fathers talk about Baptism and Confirmation and Eucharist in terms that make it difficult to see how one could give a moral-causality interpretation of what they say (and, in fact, it's not uncommon for moral-causality theorists to make a special exception for the Eucharist due to the doctrine of Real Presence). It seems that the easiest path here is to be pluralist: take Matrimony and Reconciliation to work by moral causality and the rest to work by physical causality. This loses a nice unified account, but it would make some sense for the sacramental causality to work by both moral causality and physical causality.
Nonetheless, while it's certain that each sacrament has a moral causality (they can all be seen as a kind of prayer, for one thing. and beyond whatever they do themselves they each may also be offered up in prayer for further grace), I think physical-causality theorists should hold the line, and hold that all the seven sacraments properly work by physical causality. This does raise some questions, that have never adequately been answered, about Matrimony and Reconciliation/Penance. (Perhaps relatedly, Thomas Aquinas, who does very well in the Summa Theologiae in expounding a plausible version of what later came to be called the physical causality view, never completed his discussion of Penance and never got to Matrimony. What he does say about Penance suggests that he was thinking of the human beings involved as the sensible instruments, and thus God working inwardly in them gives the physical causality for grace to them. As far as I know, no one has ever really developed this.)
There are some very solid reasons for being a physical-causality theorist. The Tridentine formula for sacramental causality is that the sacraments contain and confer grace, and while moral causality theorists give us at least a roundabout sense of 'confer', they tend to go quite squishy and difficult to pin down when they talk about the 'contain'. Melchior Cano claims that the sacraments contain grace 'morally' -- which seems to equivocate on 'morally' again -- in the way that a purse filled with gold contains the price of a ransom. It's very difficult to figure out what this means, although this is not wholly Cano's fault -- he seems to have the idea that grace is contained in the sacrament by way of a sort of designated status of some kind (a purse holds the price of a ransom purely because the gold inside is designated to fulfill an already existing function of paying a ransom), and social ontology is a philosophical tangled field. But in the purse example, the price of ransom is contained in the purse only because the gold designated for the price is already, and independently, literally contained in the purse; there isn't obviously anything like this in Matrimony or Confirmation. (Cano claims that the thing contained is the blood of Christ, which is unilluminating when you want to know how.) Louis Billot tried a different route, holding that a sacrament is a title for the right to grace; but it's equally unclear what it means to say a title 'contains' that to which it gives right, since we don't normally talk about (say) the deed of a house 'containing' the house, and if we did it would almost certainly be a metaphor for something else. Moral causality externalizes the actual causal work of the sacraments, since moral causes only induce a physical cause to act, which makes it hard for moral causality theorists to say how the grace could possibly be said to be contained in the sacrament, since it is in some sense caused outside of the sacrament. Physical-causality theorists have no problems with this at all. And nothing prevents a physical-causality theorist from also recognizing that the sacraments have a designated status and are capable of being juridical titles within the broader covenantal framework, beyond their causality for their proper effects.
