Hume stated in his An Enquiry Concerning Human Understanding, “In vain…should we pretend to determine any single event, or infer any cause or effect, without the assistance of observation and experience.” It is precisely in this psychological claim that Hume developed a lens to view the relationships governing necessary connection and cause and effect among objects in the external world. Metaphysically, Hume may recognize the objective existence of material objects, but when it comes to the relationships controlling these objects and the role that the natural world plays in governing such relationships, Hume creates a framework in which these relationships are not explained by our direct knowledge of objective, natural laws and their influence on the objects, but rather through something that is completely inside of our heads. Through an analysis of Hume’s characterization of how the human mind perceives and interprets causal events and connections occurring among objects in the external world independent of our senses, I will demonstrate that the causal laws governing Humean metaphysics are completely dependent upon, and can only be explained by, Hume’s psychological claims.
Before I begin an analysis of Hume’s psychological views concerning the relationships between external objects and how this makes up the foundation of his metaphysics, let me first expand upon two primary reasons for such a strong claim regarding the complete dependence between the causal laws governing Hume’s metaphysics and his psychological explanations. For this not to be the case, that is, for the causal laws governing Humean metaphysics to not completely depend upon Humean psychology, the criteria for two things would need to be met - namely, that Hume has to recognize that objective, natural laws exist and govern the relationships between material objects and that we are capable of fully perceiving and knowing these laws such that we are able to make inferences about cause and effect among objects based on our absolute knowledge of these laws. This is because, if an objective, causal law were to exist, the existence of that objective causal law would necessarily be independent of Hume’s psychology, by definition of what it means for something to be objective. However, we still require Hume’s psychological claims to perceive and understand any objective or non-objective causal laws. In this way, the existence of any objective causal laws would necessarily be independent of Hume’s psychology, but any instance of a perception/understanding of an objective or non-objective causal law with the mind would not be able to occur without Hume’s psychological mechanisms.
As we will see, the first criteria is addressed by Hume at a few points in his work, but we are ultimately left with an ambiguous conclusion about such matters, and in regards to the second criteria, Hume essentially argues that humans are incapable of ever knowing metaphysical governing laws a priori, and even then, we can only develop our own subjective views about the relationships governing metaphysical objects through experiences completely dependent upon our individual psychology. If I may, I would like to analyze these two essential points in reversed order to show the dependent relationship between the causal laws of Humean metaphysics and psychology.
The primary component that gives rise to a Humean metaphysics whose laws are explained by psychological claims starts off with objects of the human mind concerning “matters of fact,” or things concerning our observations and inferences of real world objects, which is primarily founded upon the phenomenon of “Cause and Effect.” Let’s use the example of some object B following from some object A for this analysis. Hume begins to build up his argument by stating, “All reasoning of matters of fact seem to be founded on the relation of Cause and Effect,” (Section IV, [4]) recognizing that there is some sort of relationship between objects A and B. How does this relationship between objects A and B arise? Hume goes on to say, “I shall venture to affirm, as a general proposition, which admits of no exception, that that the knowledge of this relation is not, in any instance, attained by reasonings a priori, but arises entirely from experience, when we find, that any particular objects are constantly conjoined with each other.” (Section IV, [6]) In other words, Hume is stating that we do not deduce that a B will follow from some A by theoretical reasoning prior to experience, but that we know that a B must follow from an A because of our experience of a B typically following from an A in the past. No where does Hume state that we are making inferences of cause and effect based on our direct observation of the laws governing the relationship between material objects. In fact, he later says, “No object ever discovers, by the qualities which appear to the senses, either the causes which produced it, or the effects which will rise from it; nor can our reason, unassisted by experience, ever draw any inference concerning real existence and matters of fact.” (Section IV, [6]). To Hume, our entire conception of cause and effect occurring between objects is rooted in a purely psychological mechanism: the mind’s experience of cause and effect. However, just because we cannot make the connection of cause and effect without the psychological mechanism of memory and experience does not mean that the relationships of cause and effect are something that the mind creates in and of itself rather than a relationship existing between external objects independently of our minds that we are perceiving. To see that the metaphysical laws such as cause and effect between external objects is an entirely psychological phenomenon, let’s examine what Hume has to say about using experience as a basis for inference concerning future events of cause and effect.
Regarding experience, Hume goes on to state, “But why this experience should be extended to future times, and to other objects, which for aught we know, may be only in appearance similar; this is the main question on which I would insist.” (Section IV, [16]). If objective laws of nature did indeed exist, and we were capable of perceiving and understanding them when we experience cause and effect, then why can’t we take these general natural causal laws and extend them to future events of cause and effect? Furthermore, Hume seems to be skeptical of relying on experience itself to form inferences concerning cause and effect, as he says, “If there be any suspicion, that the course of nature may change, and that the past may be no rule for the future, all experience becomes useless, and can give rise to no inference or conclusion.” (Section IV, [21]) To Hume, not only are we incapable of ever drawing any inference concerning “real existence and matters of fact” without consulting our experience, but even then, experience itself can give us no definite conclusion about the past being a rule for the future if there is even a slight suspicion that nature may change. Hume is essentially recognizing that human beings base all of their inferences on a shaky psychological foundation known as experience, which is not something that encapsulates the laws of nature itself, but our own subjective laws that we have formed through repeated exposure to the relationships between external objects. Indeed, if there was any suspicion that “the course of nature may change,” we would still be able to make accurate inferences about metaphysical relationships in the case that our knowledge of these relationships was not derived from the psychological mechanism of experience, but was instead derived from direct knowledge about the laws of nature and the way that nature operates. It appears, then, that not only is our knowledge of metaphysical relationships rooted in our psychological realm of experience, but this psychological realm itself is severely limited in its application towards inferences about future events in the world. We will see, in fact, that the subjective relationships between cause and effect concerning objects rooted in our experience may not be logical “relationships” at all, and certainly not direct knowledge of the natural laws of cause and effect, which may not actually exist.
Hume believes that any inferences we make about operations in the external world that are derived from our psychological experience are not inferences which are based upon any logical reasoning, and it is at this point that we are introduced to custom and conjunction to make the relationship between governing laws in Humean metaphysics and psychology even clearer. Hume states, “For wherever the repetition of any particular act or operation produces a propensity to renew the same act or operation, without being impelled by any reasoning or process of the understanding; we always say that this propensity is the effect of custom…All inferences from experience, therefore, are effects of custom, not of reasoning.” (Section V, [5]) Hume is saying that it’s only by custom, or in other words, by us continuously experiencing the same phenomenon over and over again throughout our lives, that we automatically assume that a B will follow from an A. There is no logical explanation we can come up with for a B following an A, other than the fact that it seems to have always been the case that a B followed from an A in our experience. Furthermore, Hume says “All belief of matter of fact or real existence is derived merely from some object, present to the memory or senses, and a customary conjunction between that and some other object.” (Section V, [8]), further emphasizing his point that, although originally derived from the real existence of objects B and A, beliefs concerning the relationships between matter of fact such as an object B following from an object A are created in our minds from the conjunction of these two objects in our experience – once again, a purely psychological phenomenon created in the presence of external objects. At this point, it should be clear that the metaphysics concerning the relationships between external objects cannot exist without explanation from Hume’s psychological theories. What about the qualities of power and necessary connection, another realm of governing laws in Humean metaphysics? Can we acquire direct knowledge of these in physical objects without a psychological explanation built upon the framework of subjective experience on Hume’s behalf?
According to Hume, we cannot. Hume says, in fact, “When we look about us towards external objects, and consider the operation of causes, which are never able, in a single instance, to discover any power or necessary connexion; any quality, which binds the effect to the cause, and renders the one an infallible consequence of the other.” (Section VII, [6]) In other words, upon viewing a single instance of a B following from an A, there is no apparent connection between A and B, nor is there any apparent power in A to produce a B. Hume goes as far as to state, “But were the power or energy of any cause discoverable by the mind, we could forsee the effect, even without experience…”(Section VII, [6]) Once again, we are brought into a philosophy where knowledge of metaphysical relationships between objects cannot exist without the psychological mechanism of experience of the conjunction of two objects. We cannot make an inference about any effect from a cause because we simply cannot directly perceive the power or energy of any cause from its effect. So in Hume’s view, and as previously explained, our subjective ideas of power and necessary connection arise from the experience of the conjunction between two objects in our experience. We may say that the two objects A and B are connected, or that A has the power to cause B, but in reality, this is nothing more than us assigning labels concerning the relationship of two objects in our experience which happen to be frequently “connected.” In this sense, it is incoherent to talk about the causal relationships governing objects independent of the human mind, as the human mind is fully responsible for deducing these subjective laws of cause and effect, power, and necessary connection which are nothing more than phenomenon existing solely in our experience and the contemplation of said experience– not in the objects themselves. To further expound upon the limitations of the human mind and show that necessary connection and power are nothing more than illusions of our experience, or in other words, that Hume’s metaphysical relationships are dependent upon his psychology, Hume explains his case using the copy principle.
The essence of the copy principle according to Hume, is that any thought that we have can be traced back to some physical impression. Regarding our experience of a B following from an A, Hume states, “This is the whole that appears to the outward senses. The mind feels no sentiment or inward impression from this succession of objects: Consequently, there is not, in any single particular instance of cause and effect, any thing which can suggest the idea of power or necessary connexion.” (Section VII, [6]) What Hume is trying to say, in other words, is that the succession of objects does not imprint inside of our minds the power or necessary connection between the objects – we simply see that the succession of one follows from the other. Where do our ideas of power and necessary connection come from, then, since they cannot be traced back to any impression? The answer is that they come from our experience alone, as Hume later says regarding the volition of our will to produce movement in a limb, “We learn the influence of our will from experience alone. And experience only teaches us, how one event constantly follows another; without instructing us in the secret connexion, which binds them together, and renders them inseparable.” (Section VII, [13]) So there is no distinct impression of power or necessary connection between external events – we only get an idea of such things by reflecting upon our experience of the conjunction of objects, and then label the two objects as being connected, as explained previously. Hume then concludes with the following: “When we say, therefore, that one object is connected with another, we mean only, that they have acquired a connection in our thought, and give rise to this inference, by which they give proofs of each others existence…” (Section VII, [28]) In Hume’s own words, we can conclude the metaphysical connection between external objects may or may not exist, but that is not the point – the point is that we are the ones who create these relationships because of the connection between the objects in our thoughts.
It has now been fully explained that the relationships governing Humean metaphysics cannot exist without Hume’s psychological explanations, but what about the first criteria mentioned earlier that needs to be met in order for the laws of Humean metaphysics to not completely depend upon Humean psychology – Do objective natural laws exist, and if so, are we capable of fully perceiving and understanding them? Hume seems to think that these laws do exist indeed, as he most notably states, “Nature will always maintain her rights, and prevail in the end over any abstract reasoning whatsoever.” (Section V, [2]), but this then begs the question: if nature does have “rights,” or its own governing laws in other words, how are we (or Hume) capable of knowing that they exist? Hume even states previously in his work, “It must certainly be allowed, that nature has kept us at a great distance from all her secrets, and has afforded us only the knowledge of a few superficial qualities of objects; while she conceals from us these powers and principles, on which the influence of these objects entirely depends.” (Section IV, [16]) How does Hume know that these “powers and principles” even exist if nature is concealing them from us? The answer is that we cannot know and neither can Hume, and in Hume’s own arguments, all we can say for sure is that we deduce that objects A and B have a connection between each other simply because in our experience, they are frequently conjoined – any attempt at recognizing whether metaphysical laws between objects exist independently of the our psychological experience is pure speculation and territory of ambiguity in Hume’s framework.
Although it is not the case that all of Humean metaphysics is dependent upon Humean psychology (for example, it is the case that Hume recognizes the existence of material objects independent of our senses, unlike Berkeley), it can be said conclusively that the relationships of the causal laws governing Humean metaphysics are completely dependent upon, and can only be explained by, Humean psychology. Perhaps this is why Hume chose to redefine what cause is in the context of experience when he said, “…an object followed by another, and whose appearance always conveys the thought to that other.” (Section IV, [29]) To Hume, governing metaphysical laws such as cause and effect, power, and necessary connection between physical objects may or may not exist in nature, whom apparently conceals such things from us, but instead only make sense on the subjective foundation of the Humean psychology of experience, custom, and conjunction. Hume said it best when explaining that single instances of the operation of bodies do not show us power or necessary connection immediately, but rather it is in the continuous appearance of such instances that “…we then begin to entertain the notion of cause and connexion.” (Section IV, [30])