Causation: Invisible Cement
I wish to make some inconclusive but perhaps suggestive remarks about a subject that has troubled me for 50 years. Hume’s imperishable contribution is well known: whatever the objective nature of causation may be, we can say with confidence that our knowledge of it is neither a priori nor observational. We don’t know how objects will causally influence each other by pure reason (causal statements are not analytic) and we don’t perceive causal relations (particularly causal necessity). When we ascribe a causal power to a thing we don’t do so because of a rational insight (as in mathematics) and we don’t do so because we can literally see the power (as we can see color and shape). So how do we know the causal powers of a thing—how, say, do we know the effects of a moving billiard ball on a collision course with another billiard ball? According to Hume, we know it by induction: we have seen other objects behave in certain ways and we can see that this object is similar to those. Of course such reasoning is inconclusive and fallible, as well as being annoyingly indirect, but it is the only method we have. I can know empirically that a billiard ball is red by looking at it and I can know a priori that it is extended because I know it is a physical thing, but I can’t know what it has the power to do by either method; I must resort to my recollection of how similar balls have behaved in the past and trust that it will follow suit, knowing that this reasoning is far from satisfactory. The causal power is curiously elusive, frustratingly hidden from view: it exists right there in the object just waiting to unleash itself, and yet I cannot get my mind around it. I can stare at the object all day from every angle and in every light, but the power will not reveal itself to me. If causation is the cement of the universe, it is invisible cement—I can get no impression of it. The primary and secondary qualities of the object display themselves without this coyness, but causal powers lurk unseen, refusing to show their face. Thus the epistemology of causation is subject to skepticism, to the point that causation itself comes into question.
The question I want to ask is why it is thus invisible: what is it about causation as it exists in objects that makes it closed to perception? What is it about the nature of causation that explains its imperceptibility? It can hardly be that it is too small or has the privacy of the mind or is a type of value or is abstract like numbers—none of these possible explanations applies in this case. It must somehow be intrinsically hidden from the senses, and thus only known (if at all) by the indirect method of induction, but what is its nature such that this is so? I suggest it is because causal powers relate essentially to the future; they determine how an object will behave. An object with a certain power is such that in certain conditions it will bring something about at a later date: the speeding billiard ball will have certain effects in the future as it collides with other balls. The power is future-oriented, future-involving. Yet it is possessed presently: it is now true that this object will do that. But I can only observe what is present—my senses don’t take in the future (as they don’t take in the past). It is as if a certain kind of “externalism” applies to the power: it is what it is now in virtue of what it will be later, i.e. something external to the present time. It has a trans-temporal quality, a foot in the present and a foot (many feet) in the future. It is temporally spread out. But then it will not be perceptible by the senses, since they take in only what is present (perceptions are themselves caused by present facts). If we possessed precognition as a type of sense, then we would be able to see the causal powers of objects and would not need to rely on extraneous (Hume’s word) induction; but as it is we are blind to causal powers, being forced to look outside the particular causal nexus before us and go by past similarities. If this is right, it is at least clear why causation is invisible: it is invisible because we can’t see the future. We can know the future by means of truths of reason—we know that in the future bachelors will always be unmarried—and we can know it by induction (putting aside skepticism)—we know that the sun will rise tomorrow based on its past performance—but we don’t know it by perceiving it directly. So causal powers elude our powers of perception, since they intrinsically include future happenings.
Powers are a bit like biological functions. The heart has the function of circulating the blood, but this involves future exterior events, so we can’t see its function by looking at it, as we can see its gross anatomy. We can’t look at it at a given time and simply observe that it has that function. It is possible for a heart to have this function and not have the power to carry it out (it is a defective heart) and vice versa, so these are not the same property; but they share the characteristic that they incorporate “external” events. We can see the heart but not see its function, as we can see a billiard ball but not see its causal powers—because in both cases the property in question “takes in” remote occurrences. In the case of powers this remoteness is temporal: the power is defined precisely as the power to change the future in a certain way. Powers are cross temporal not temporally confined—what it is to be a causal power is not constituted by what holds at a given instant (like color or shape). Powers reach into the future, but perception is stuck in the present. That is why Hume is right bout the epistemology of causation. It is thus not mysterious that causal powers should be invisible; it stems from their very nature. It may be that this nature is itself mysterious, but given its reality imperceptibility can be predicted. As remarked, pre-cognition (pre-perception) would reveal it to the mind, but we don’t possess that—and it may be conceptually impossible. In any case our senses do not put us into epistemic contact with the future—only inductive reasoning can do that (or knowledge of analytic truths). Causal powers are something additional to the usual primary and secondary qualities, and different in nature: for they are not temporally confined. This implies that you can’t see causal powers, because you can’t see what an object will do.
It is arguable that all causes and effects involve motion so that causal powers are always powers to move things (strictly, accelerate things). If so, the future changes that are implicit in present powers are changes of motion, which means that what we can’t perceive are future motions when we fail to perceive powers. We have no perception of future motions; we can only perceive present motions. That is precluded by the fact that there is no backwards causation, so no present perception can be caused by a future event. Future motions can’t cause present perceptions of them because perceptions must be caused by what they are of and there is no backwards causation. Hence we cannot perceive powers to produce future motions. The temporal asymmetry of causation lies behind our inability to perceive causal powers. If backwards causation were possible, then we would be able to perceive future motions, and hence perceive powers; but we can’t do the latter because the former is not possible (not in the actual world anyway). So the unobservable character of causation has its roots in the (actual) impossibility of backwards causation. Hume’s critique springs from that fundamental fact. It is not that there is some (wholly) present fact about the cause that for some reason we can’t perceive; it is the involvement of future facts that stands in the way of such perception. In vain do we look for future motions in the perceived world. Therefore what we observe of causes does not include their being the causes they are. To put it another way, the universe unfurls towards the future as causal powers manifest themselves, but we have no perception of this future, so the powers remain invisible to us. Metaphorically speaking, the universe knows the future better than we do. If we lacked the ability to make inductions based on similarities, we would have no causal knowledge at all; we would not even have a conception of causation. For us history would be just one damn thing after another. It would not be apparent to us that causal powers exist, even though they do, since nothing in what we perceive can give us the idea of them. Perception and causation are just not cut out for each other, given what each of them intrinsically is. They relate quite differently to time. Perception is directed exclusively to the present while causation is inherently future-directed.
 It should be noted that this applies to mental causation as well as physical causation: we don’t introspect mental causal powers, as we don’t perceive physical causal powers. Hume, of course, was well aware that his critique applies as much to mental causation as physical causation. I don’t believe he ever asked why causation is not revealed to the senses, being content to note that it isn’t.