In Plato’s allegory of the cave, he describes a scene where a group of individuals is trapped in a cave facing away from the source of light. Meanwhile, all sorts of objects and people parade on a raised path, casting their shadows on the wall they can see. For these cave dwellers, their knowledge of reality includes only those shadows since they are the only sensations available to their senses. The situation these unlucky cave dwellers find themselves in anticipates the metaphysical dilemma that Hume tackles in his A Treatise on Human Nature. Hume develops a skeptical approach to examining the external world: how is it that one knows the nature of objects when all one has at their disposal is perception? If perception is the constant mediator between any external realities and assumptions made about objects and causes, is it possible to develop any knowledge about reality? In parsing these questions, Hume concludes that there must be a pernicious “skepticism” with regards to all previously understood absolutes, including the nature and causal properties of the external world.
To consider the rest of Hume’s argument, it is necessary to accept his premise that there is not an infallible relation between what is perceived and what exists in the external. Hume brushes aside the possible argument that there may be no external bodies whatsoever by commenting “that is a point, which we must take for granted in all our reasoning” (Hume 187). In doing this, Hume dodges the bullet proposed by Bishop Berkeley, where nothing exists at all and reality exists under deceived perception (Fogelin, 4). Instead, he chooses to focus on the question “what causes induce us to believe in the existence of a body” (Hume 187)? If one assumes that perceptions are indeed limited, the question becomes how can we gain any substantive knowledge at all about the outside world?
The senses are not capable of providing an individual any more information than what is directly around him. Hume proposes that senses “convey to us nothing but a single perception, and never give us the least intimation of anything beyond” (189). In other words, human perception is by definition limited to those things which one can perceive; senses do not deliver any more information than sensations. Therefore, anything beyond the limited sphere of our senses at any given moment cannot be validated or invalidated at the level of perception. This paradox demonstrates the difficulty of making extrapolations from what we can perceive, or as Hume writes, “We ought not to receive as reasoning any of the observations we may make concerning identity, and the relations of time and place; since in none of them the mind can go beyond what is immediately present to the senses” (73). A way to visualize this would be to imagine a video camera fixed in one direction broadcasting to a remote location. With no way to turn the camera around, the remote observer would have no way to tell anything about the area where the camera was not pointing. It is impossible as a matter of fact to make any assumptions about the area you cannot see.
Accepting the premise that perception is limited, Hume proceeds to inquire how individuals know that objects continue to exist when they are not readily available for us to perceive. Since perception is the only way in which we take in the world, how do we know that objects are external and that they are not annihilated when they lie outside of the sphere of our perception? Hume posits that “philosophy informs us, that everything, which appears to the mind, is nothing but perception and…is dependent on the mind” (193). Therefore, it becomes necessary to prove that these objects are not internal only, but have an external manifestation and do not poof out of existence when we glance away. This point brings to mind the age-old dilemma—of a tree falls in the forest, does it make a sound?
To demonstrate this point, Hume uses the example of a porter arriving in his study to deliver a message from a friend abroad (196). Although his senses only extend to what is in his room, he can make certain inferences from indirect action. For example, even though he doesn’t see the door open directly, it can be assumed from the fact that he heard the sound of the door, and that sound is only associated with the door opening, that it did indeed open even though he didn’t perceive it. Similarly, even though he has no immediate perception of his friend or the country that he is in, by his perception of the letter, he can assume that they exist, as well as the ferry and the mail system.
Hume suggests that the mechanism on which individuals base these assumptions is the imagination. He writes that “since all impressions are internal and perishing existences…the notion for their distinct and continued existence must arise from a concurrence of some of their qualities with the qualities of imagination” (194). In other words, because the perceptions of objects only last so far as they are in our observable sphere, the mental construct of the imagination allows individuals to maintain an understanding of some perceived objects as continuous. Hume lays out several criteria for this to happen: “all those objects to which we attribute a continued existence, have a peculiar constancy” (194); “[these objects] preserve a coherence, and have a regular dependence on each other…and produces the opinion of their continued existence” (195). In instances where fleeting perceptions are both constant and coherent, the imagination allows the individual to assume that these objects exist independently of his mind, and continue to exist without his help.
Perceptions, therefore, make an impression upon memory and the imagination which becomes the foundation for conclusions that individuals make in regards to the outside world. Hume argues that it is out of those perceptions that we form ‘the mind’: “what we call a mind is nothing but a heap or collection of different perceptions which constitute a thinking being” (207). In simple terms: “all ideas are derived from impressions” (160). Ideas do not come about because they are directly perceived externally, but rather are mental constructs that are based on the information and inferences made from perception. This is a dramatic realization because it indicates that all knowledge beyond what is temporarily validated by perception is the creation of the mind and not something that exists externally.
Using this argument, Hume tackles the notion that some of these metaphysical questions could be answered by deferring to a higher power. For example, “Cartesians proceeding on their principle of innate ideas, have had recourse to a supreme spirit or deity” (160). The Cartesians, in a sense, dodge the question by injecting a supreme being. However, Hume points out that “if every idea is derived from an impression, the idea of a deity proceeds from the same origin” (160). Therefore, Hume proceeds to argue, “the supposition of a deity can serve us in no stead” (160) in dealing with the complicated notions of perception and causation.
Furthermore, by making the distinction between the world as it exists in reality and the world that exists at the level of mental constructs, Hume differentiates between what he terms “vulgar” and “modern” philosophy. The “vulgar” system “suppose their perception to be their only objects and at the same time believe the continued existence of matter” (209). This system proposes a one-to-one relationship where everything that is perceived is believed to be real and the only standard for reality. However, Hume argues for the more advanced, modern philosophical view of a “double existence” which distinguishes “betwixt perceptions and objects of which the former are supposed to be perishing and interrupted at every turn” (211). Again, Hume points out that there is a fundamental difference between what may exist in the real world and our perceptions and conclusions about it. It is entirely possible that in this double existence, both versions will be the same; however, it is that uncertainty which Hume asks the reader to consider as a source for skepticism.
Under what circumstance could an individual proceed to make the judgment that items in the outside world have a causal relationship? Hume defines cause as “an object precedent and contiguous to another, and so united with it, that the idea of the one determines the mind to form the idea of the other, and the impression of the one to form a more lively idea of the other” (170). Cause is not directly observable to our senses. Hume writes that causation “can be traced beyond our senses, and informs us of existences and objects, which we do not see or feel” (74). For example, if two billiards balls collide, there is nothing that exists at the level of perception which indicates that the first ball caused the second to move. However, we assume that this was the case as a result of experience; “Cause and effect are relations, of which we receive information from experience, and not from any abstract reasoning or reflection.” (69). If one drops their copy of Hume’s Treatise of Human Nature, experience informs the result of it falling to the ground. This is apparent because, in previous instances of objects that have been dropped, a similar result occurs. Hume contends that “all our arguments concerning causes and effects consist both of an impression of memory or senses, and of the idea that existence, which produces the object of the impression or is produced by it” (84) Individuals believe in cause and effect because they have seen it take place by course of habit and repetition, and not for any reasoned or logical cognition.
Hume uses the example of billiard balls being thrown at each other in one instance, and then again in twelve months. Even though the effect observed in the first instance has no bearing on the second instance, as a matter of habit, individuals assume that the reaction will be the same. As Hume puts it “these impulses do not influence each other. They are entirely divided by time and place” (164). In other words, since these two instances are entirely separate events, it is simply a matter of habit, and the operation of the imagination, which causes one to take for granted that the reaction between the billiard’s balls will be the same. While this may or may not be the case, Hume’s point is that we have no real method for knowing if this is the case due to the limited nature of our perception. Causation manifests itself not in the external world as a result of the properties of objects, but internally as a result of imagination: “When I cast my eye on the known qualities of objects, I immediately discover that the relation of cause and effect depends not in the least on them” (77). In observing two objects which appear to affect a causal relationship on each other, that perception does not depend on anything inherent in the objects. Rather, it is at the level of perception in which the connection of a causal relationship is made.
However, Hume doesn’t deny that causes exist but rather suggests that someone of limited senses can’t know what those causes might be. In part III, section iii Hume addresses the issue of “why a cause is always necessary.” He begins by observing that “’tis a general maxim in philosophy that whatever begins to exist, must have a cause of existence” (78). He proceeds to argue that this is not something that can be proven in anything but a circular fashion: “when we exclude all causes, we do exclude them, and neither supposes nothing nor objects itself to be the causes of the existence.” Yet, he does eventually conclude that there is no way to deny that causes do not exist but rather demonstrates that “since it is not from knowledge or any scientific reasoning that we derive the opinion of the necessity of a cause to every new production, that opinion must necessarily arise from observation and experience” (82). Hume is not denying that there are causes or causation, but he is questioning the basis on which individuals would come to understand those causes.
Taking all of Hume’s arguments collectively, there is reason to be ‘skeptical’ about not only the outside world but all of the deeply held convictions or ideas that have been accepted as general truths. If, as Hume has demonstrated, there is no way to understand infallibly the relations because objects and ourselves, or between external bodies, then how are we to judge laws of science and philosophy? Any inferences made must be taken as a matter of probability and not as a matter of fact. Hume writes that “this skeptical doubt, both concerning reason and the senses, is a malady, which can never be radically cured but must return upon us every moment” (218).
In other words, on all subjects, there must permanently exist a shadow of doubt due to the limits of our perception and possible failure to understand what is taking place. Fogelin suggest Hume demands “a recognition that all our inferences beyond the present or experience derive from [perception and impressions] may or may not make us more skeptical concerning them, but this discovery, at the very least, deflates our intellectual pretensions by revealing that some of our most important modes of inference are made in the complete absence of rational insight” (Fogelin, 10). Hume presents a system of uncertainty and incomplete information. The question that shakes our fundamental beliefs to the very core: “what can we look for from this confusion of groundless and extraordinary falsehood? And how can we justify to ourselves any belief we repose in them?” (Hume 218)
There are some points that Hume possibly takes for granted which may serve to undermine his argument. For example, if his argument is indeed true, and there is no way to know if something is or is not true, on what basis can he make this argument? Hume is aware of this potential paradox when she writes, “I rely entirely upon [sense and reason], and take it for granted” (218). Unfortunately, due to the nature of philosophy, Hume is required to use the very tools of cognition that he seeks to prove as potentially misleading. For example, Hume uses several anecdotal explanations for his ideas. If the factual basis for these stories was disproved because the causation of the events was not correctly perceived then they would be misleading at best. Put another way, how can Hume be certain that his philosophy is not “groundless” and based on “extraordinary falsehood”? Indeed, if Hume’s philosophy is correct then the reader is left with absolutely no way to tell.
Furthermore, Hume accepts that individuals experience the world through the medium of perception, but brushes aside the argument made by Bishop Berkley (Fogelin, 4) that perception may be entirely misleading, and there is no outside world whatsoever. Hume seems to suggest that this factor is irrelevant because it can neither be proved nor disproved; however, if there was no outside world, then there would be no basis for determining that objects were external to the observer. Indeed, Hume seems only to go have way in his critique on perception, suggesting that it is fallible, but not perfectly fallible.
The poor cave dwellers who were shackled into a life of limited observation in Plato’s Allegory may have been equally utilized to describe Hume’s theory of ‘skepticism.’ As they were trapped within the confines of a cave, we are trapped within the confines of our perception. Our observations create impressions on our imagination, which when stitched together by habit form ideas and the notion of causation. Hume doesn’t try to argue against the likelihood that there are external causes but instead suggests that we must be ‘skeptical’ when we think he has found them. According to Hume, there must always be a shadow of a doubt that what we believe to be happening externally is reality and not simply a mental fabrication.
Fogelin, Robert J. “Hume’s skepticism.” The Cambridge Companion to Hume. Ed. David Fate Norton. Cambridge University Press, 1994. Cambridge Collections Online. Cambridge University Press. Web.
Hume, David. A Treatise of Human Nature. Vol. 1. London: Oxford UP, 1941.