David Hume took the genuinely empirical elements in the philosophy of Locke and Berkeley, rejected some lingering metaphysics from their thought, and gave empiricism its clearest and most rigorous formulation.
Hume’s most original and influential ideas deal with the problem of causality. Neither Locke nor Berkeley challenged the basic principle of causality. Although Berkeley did say that we cannot discover efficient causes in things, his intention was to look for the cause of phenomena and therefore the predictable order of nature in God’s activity.
For Hume, the very idea of causality is suspect, and he approaches the problem by asking the question, “What is the origin of causality?” since ideas are copies of impressions, Hume asks what impression gives us the idea of causality. His answer is that there is no impression corresponding to this idea. How then does the idea of causality arise in mind? It must be that the idea of causality arises in the mind when we experience certain relations with objects.
When we speak cause and effect, we mean to say that A causes B. but what kind of relation does this show between A and B? experience furnishes us with two relations: first, there is the relation of contiguity, for A and B are always close together; second, there is priority in time, for A, the “cause,” always precedes B, the “effect.” But neither contiguity nor priority implies “necessary” connection between objects.
There is no object, Hume says, that implies the existence of another when we consider objects individually. No amount of observation of oxygen can ever tell us that when mixed with hydrogen it will necessarily give us water. We know this only after we have seen the two together. It is by experience only that we can infer the “existence of one object from another.” While we do not have any impression of contiguity, priority, and constant conjunction, we do not have any impression of necessary connections. Thus, causality is not a quality in the objects we observe but is rather a “habit of association” in the mind produced by the repetition of instances of A and B.
In so far as Hume assumed that the causal principle is central to all kinds of knowledge, his attack on this principle undermined the validity of all knowledge. He saw no reason for accepting the principle that whatever begins to exist must have a cause of existence as either intuitive or capable of demonstration. In the end, Hume considered thinking or reasoning “a species of sensation,” and as such our thinking cannot extend beyond our immediate experiences.
Hume’s extreme empiricism led him to argue that there is no rational justification for saying that bodies or things have continued and independent existence external to us. Our ordinary experience suggests that things outside of us do exist. But if we take seriously the notion that our ideas are copies of impressions, the philosophical conclusion must be that all we know is impression. Impressions are internal subjective states and are not clear proof of an external reality. To be sure, we always act as though there is real external world of things, and Hume was willing to take for granted in all our reasoning that things do exist. But he wanted to inquire into the reason why we think there is an external world.
Our sense do not tell us that things exist independent of us, for how do we know that they continue to exist even when we interrupt our sensation of them? And even when we sense something, we are never given a double view of it whereby we can distinguish the thing from our impression of it; we have only the impression. There is no way for the mind to reach beyond impressions or ideas they make possible.
Our belief that things exists external to us, Hume argues, is the product of our imagination as it deals with two special characters of our impressions. From impressions our imagination becomes aware of both constancy and coherence. There is constancy in the arrangement of things, when, for example, I look out of my window: There are the mountains, the house and the trees. If I shut my eyes or turn away and then later look at the same view again, the arrangement is still the same, and it is this constancy in the contents of my impressions that leads my imagination to conclude that the mountains, the house and the trees exist whether I think of them or not. Similarly, I put a log on the fire before I leave the room, and when I return it is almost in ashes. But even though a great change has taken place in the fire, I am accustomed to finding this kind of change under similar circumstances.
In case of mountains, there is constancy in our impressions, whereas in respect to the fire our impressions have a coherent relation to the process of change. For these reasons, the imagination leads us to believe that certain things continue to have an independent existence external to us. But this is a belief and not a rational proof, for the assumption that our impressions are connected with things is “without any foundation in reasoning.” Hume extends this skeptical line of reasoning beyond objects or things to consider the existence of the self, substance and God.
The only way, Hume says, to solve the problem of disagreements and speculations regarding “abstruse questions” are to “enquire seriously into the nature of human understanding, and show from an exact analysis of its powers and capacity, that it is by no means fitted for such remote and abstruse subjects.” (Johnson, p.121) Accordingly, Hume carefully analyzed a series of topics that led him to skeptical conclusion, beginning with an account of the contents of the mind.
Nothing seems more unbounded than human thought according to Hume. Although our body is confined to one planet, our mind can roam instantly into the most distant regions of the universe. Nor, it may seem, is the mind bound by the limits of nature or reality, for without difficulty the imagination can conceive the most unnatural and incongruous appearances, such as flying horses and gold mountains. But, though the mind seems to possess this wide freedom, it is, Hume says, “Really confined within very narrow limits.” In the last analysis, the contents of the mind can all be reduced to the materials given us by the sense and experience and those materials Hume calls perceptions. The perceptions of the mind take two forms, which Hume distinguishes as impressions and ideas.
Impressions and ideas make up the total content of the mind. The original stuff of thought bus an impression, and an idea is merely a copy of an impression. According to Hume, the difference between an impression and an idea is only the degree of their vividness. The original perception is an impression as when we hear, see, feel, love, hate, desire or will. These impressions are lively and clear when we have them. When we reflect upon these impressions, we have ideas of them, and those ideas are less lively versions of the original impressions. To feel pain is an impression, whereas the memory of this sensation is an idea. In every particular, impressions and their corresponding ideas are alike differing only in the degree of vivacity.
Besides merely distinguishing between impressions and ideas, Hume argues that without impressions there can be no ideas. For if an idea is simply a copy of an impression, it follows that for every idea there must be a prior impression. Not every idea, however, reflects an exact corresponding impression, for we have never seen a flying horse or a golden mountain even though we have ideas of them. But Hume explains such ideas as being the product of the mind’s faculty or compounding the data provided by our senses and experiences.
If we have any suspicion that a philosophical term is employed without meaning or idea, we need to ask where the impression was derived. And if it be impossible to assign any, this will serve to confirm our suspicion. Hume subjected even the idea of God to this test and concluded that it arises from reflecting on the operations of our own minds analyzing without limit the qualities of goodness and wisdom that we experience among human beings. But if all our ideas are from impressions, how can we explain what we call thinking or the patterns by which ideas group themselves in our minds?
It is not by mere chance that our ideas are related to each other. There must be some bond associating quality that is prevailing. It is not special faculty of the mind that associates one idea with another, for Hume has no impression of the structural equipment of the mind. But by observing the actual patterns of our thinking and analyzing the groupings of our ideas, Hume thought he had discovered an explanation for the association of ideas.
His explanation was that whenever there are certain qualities are three in number: resemblance, contiguity in time or place, and cause and effect. Hume believed that the connections of all ideas to each other could be explained by these qualities and thoughts to the original. Resemblance is mentioning one apartment in the building naturally introduces an enquiry concerning others (contiguity) and if we think of a wound, we can scarcely forebear reflecting on the pain which follows it (cause and effect).
There are no operations of the mind that differ in principle from one of these three examples of the association of ideas. But of these, the notion of cause and effect was considered by Hume to be the central element of knowledge. He took the position that the causal principle is the foundation upon which the validity of all knowledge depends. If there is any flaw in the causal principle, we can have no certainty of knowledge.
Hume denied that we have any idea of self. This may seem paradoxical, that I should say that I do not have any idea of myself. Yet here again Hume wants to test what we mean by self by asking, “From what impression could this idea be derived?” Is there any continuous and identical reality which forms our ideas of the self? Do we have any one impression that is invariably associated with our idea of “self”? Hume denies the existence of a continuous self-identity and says about the rest of humanity that “they are nothing but a bundle or collection of different perceptions.” (Johnson, p.96) How then do we account for what we think is self? It is our power of memory that gives the impression of our continuous identity.
Hume compares the mind to “a kind of theatre where several perceptions successively make their appearance,” but adds that “we have not the most distant notion of the place where these scenes are represented.
What led Hume to deny the existence of a continuous self that in some way retains its identity through time was his thorough denial of existence of any form of substance? Locke retained the idea of substances as that something, which has color or shape, and other qualities, though he spoke of it as “something we know not what.” Berkeley denied the existence of substance underlying qualities but retained the idea of spirituals substances. Hume denied that substance in any form exists or has any coherent meaning.
If what is meant by self is some form of substance, Hume argued that no such substance can be derived from our impressions of sensation. If the idea of substance is conveyed to us by our senses, Hume asked, “which of them; if by the ears, a sound; if by the palate, a taste…We have therefore no idea of substance, distinct from that of a collection of particular qualities.”