As an empiricist, Hume starts with an epistemological foundation which is essentially the same as Berkeley's, but he carries out the empiricist program without Berkeley's rationalist retention of what amounts to the innate concept (or "notion" as Berkeley called it)) of "mind" or "spirit." Thus we can say Hume's empiricism is a "pure" uncompromising empiricism. But Hume pays a high price for this purity, a price Locke and Berkeley would never have been willing to pay, for Hume's analysis of knowledge on the empiricist foundationalism he inherited from his empiricist predecessors leads him inexorably to the conclusion that knowledge of the nature of reality (i.e., any metaphysical theory at all, whether it be dualist, materialist or idealist) is impossible. This is the conclusion known as skepticism, a bitter pill for the metaphysician to swallow, but Hume was prepared to "take his medicine."

Hume's Empiricistic Analysis of the Faculty of Understanding (i.e., the "Mind"):

Hume divides the contents of the mind (all of which Descartes had called "ideas" and Berkeley had called "perceptions") into two categories:

The first are what he calls "impressions" which are our immediate sensations when we are having them,
The second are called "ideas" which are the memory's copy of impressions (note that Hume's use of the word "idea" is not equivalent to Descartes' use; what Hume calls "ideas" is a subcategory of what Descartes called "ideas").

Hume argues that the only difference between these two is degree of "vivacity": the dullest "impression" is more vivid to the experiencing consciousness than the liveliest "idea." Hume claims that every idea in the mind can only originate by copying some prior impression (the basic empiricist line), but of course Hume has to explain how imagination can create ideas of things of which we have had no experience. This leads him to distinguish complex impressions and ideas from simple impressions and ideas of which they are composed. The idea of an apple, for example, is the memory's less vivid copy of a complex impression which we have had when we have experienced an apple. That idea may be broken down into its simple components: the color, the shape, the taste, the feel. the odor, etc., of the apple. Simple ideas are just those beyond which any further analysis is impossible. While I can say that the idea of red is a component of the complex idea of an apple, that idea of "red" cannot itself be further analyzed into yet simpler ideas. the mind, through its faculty of imagination, can copy simple impressions as simple ideas and then construct compound ideas by combining simple ones to create ideas of objects of which the senses have never had any impression. Thus for example I could combine in my mind the idea I have of an apple, copied from actual impressions of its shape, its feel, its taste, its odor, etc., with the idea I have of "blue" copied from impressions of blue objects, to produce in my imagination the complex idea of a "blue apple" even though of course I have never had an impression of such a thing. Thus the imagination can create ideas of centaurs and other such mythical creatures, but all of the component simple ideas of which these complex ideas are constructed must themselves be copied from some previous impression.

Hume gives two arguments for his clam that all ideas are copies of prior impressions. The first is essentially a challenge for any potential opponent to produce an idea which allegedly can be shown not to have originated in any prior impressions. Hume claims that he can analyze any idea into simple ideas all of which originated in the mind by copying impressions actually experienced. The second argument he provides is the claim that if anyone is born with a defective sense organ such that he or she cannot have impressions of a certain kind (for example a person born deaf or blind), we find that such a person does not have any ideas of the relevant impressions (for example, sounds or colors). In fact the conclusion of this second argument does seem to be confirmed by examination of such persons.

Hume uses this empiricist platform as a method for analyzing ideas. Since every complex idea can be broken down into ultimate simple ideas, and every simple idea must be a copy of some impression(s), if we want to know what is involved in a compound idea we need only break it down into its simple ideas and ask what impression was each of these simple ideas copied from.

Hume applies this method of analysis to the idea of "causal connection" which is of course an essential idea in the universal principle of causality on which many of his predecessors had relied. We can summarize the conclusion of his analysis by saying that he concludes that the impressions we have of "causes" and "effects" cannot give us any impression which is the origin of the idea of "necessary connection" between "cause" and "effect" which is claimed by all causal principles, and thus that no causal principle can ever be known. It is because of this conclusion that he ends in skepticism. What follows is an analysis of how he gets to this conclusion.

Two Kinds of Judgments (i.e. "Statements" or "Propositions"):

Impressions and ideas cannot be considered true or false by themselves; however, when combined to make assertions or "judgments" (or in more contemporary terminology, "statements" or "propositions") then the question of the truth or falsity of the judgment can be raised.  Since "knowledge" by definition must be true, knowledge, if there is any, must be composed of judgments. The first step is to consider the sorts of "judgments" of which knowledge might consist. Knowledge for Hume, as for any empiricist, consists of judgments based on our impressions and ideas which copy those impressions. These fall into two categories:

Judgments of relations of ideas are those which are such that their denial is logically impossible, e.g. "All bachelors are married." Because they are logically impossible to deny, these judgments of this type are such that their truth can be known by "reason" alone, even though, as an empiricist, Hume of course holds that the ideas on which they are based must originate in experience. Judgments which can be known by reason alone are said to be known "a priori," meaning "prior to experience." Contemporary philosophy calls judgements of this sort "analytic propositions."

The other class of judgments, judgments of matters of fact, are such that their denial is logically possible, so if they are true, they cannot be known by reason alone, but can be known to be true only after experience or "a posteriori," or as we would say today,. "empirically." These are now known as "synthetic propositions."

As long as one restricts oneself to making judgments of relations of ideas (analytic judgments) one can know these judgments are true, but only at the cost of saying nothing at all about reality. Analytic judgments are in effect just grammatical conventions or stipulations about the usages of words; their truth is, in effect, the consequence of definitions which are themselves arbitrary. For example, given the usual definitions, we can know simply by understanding the proposition that "All bachelors are unmarried" and that ""All unicorns are one-horned." This fact is expressed by saying that such judgements can be known by "reason" alone, "prior" to any particular experience of the world.  A proposition which can be known in this way is said to be known a priori. Nevertheless, from these definitions alone, which give us the meaning of "bachelor" or "unicorn," it is quite impossible to know by reason whether there are in realty (i.e., "in fact") any "bachelors" or any "unicorns" (that there are bachelors and there are no unicorns can be known, if it can be known at all, only by "experience" of the world). Thus the truth of judgments of relations of ideas (analytic judgments) does not depend on the "facts" of the world, and so while we can know "a priori" that such judgments are true, that knowledge tells us nothing about the nature of "reality." Since "metaphysics" aspires to give us knowledge of reality, metaphysical knowledge (if there is any) cannot consist of analytic judgments.

Judgments of matters of fact (synthetic propositions), however, are not true as a consequence of the definitions of their terms. If they are true, they are true because of the facts of reality. Thus if they could be known as true, such judgments would indeed be informative about the world (and so would give us metaphysical knowledge), but is it possible for us to know whether or not such judgments of matters of f act are true?

Hume's answer distinguishes between those judgments of matters of fact which are restricted to the testimony of our senses (impressions) and the contents of our memory (ideas) versus those which "go beyond" that testimony. Unless we wish to assert the metaphysical view of "solipsism" that reality simply is my impressions and ideas (and this is a view which is very probably impossible to refute), anytime we claim we know something about "the facts" or "the world" or quite simply "reality," we are "going beyond" the contents of our senses and memory. Thus (except for the solipsist) all metaphysical claims are judgments of matters of fact which go beyond the present testimony of our senses and the records of our memories.

Hume argues that any such judgments of matters of fact which go beyond the present testimony of our senses and the records of our memories requires a causal inference from what is present in our experience (impressions and ideas) as the "effect" alleged to be caused by something regarded as "outside" or beyond what we immediately experience. But for such a causal inference to be sound, it is necessary to know a causal principle which connects the effect, our impressions, to such a presumed cause in "reality" outside or "beyond" the impressions in our conscious awareness.

Hume now argues that all causal principles are such that their denials are logically possible, thus if they can be known at all, they must be known only empirically, on the basis of experience (i.e., they are all synthetic propositions or what he calls "judgments of matters of fact"). Hume sets out to show no experience can justify these sorts of principles as necessarily true. Hence his skepticism.

Hume's Analysis of Causal Inferences

First, what is a causal inference?  It is the process of reasoning from what is given as known directly by experience to something else which is not directly experienced, but which is claimed to be the "cause" of the given "effect."  From smoke seen on the horizon, we infer that there exists a fire out of sight which causes the effect of the smoke.  Any such inference must use as a premiss a "causal principle."

What then is a causal principle? Any statement of the form "C causes E" or "E is the effect of C," (where "C" and "E" are any two "objects" (Hume's term) or "events" that are called the "cause" and the "effect" respectively) expresses a causal relation to hold between C and E. Such statements as "fire causes heat" or "temperatures below freezing cause water to turn into ice" are examples of "causal principles."

But we might ask, what does it mean to say "C causes E"? Recall that Hume's empiricism leads him to hold that any idea can be explicated by analyzing it into its simple components and then showing what simple impression each component simple idea copies. The idea of a causal principle in effect includes the ideas of E following C in time and that C and E are "necessarily connected," such that when C happens, E must follow, and when E happens, C must have happened previously. Only if the two are necessarily connected would the occurrence of the cause permit one to infer that the effect has happened or will happen; or that the occurrence of the effect permits one to infer that the cause happened previously. Any process of reasoning in this way may be called a "causal inference." Thus any causal inference always requires assuming a certain causal principle to hold true, or in other words that a certain "C" and a certain "E" are necessarily connected as "cause" to "effect". This fact in turn implies that we can know a "causal inference" is a sound inference (all premises are true, so conclusion is true) only if we can know that the relevant causal principle is true.

Can we know whether any particular causal principle is true on the basis of reasoning? Hume begins by showing that there is no process of a priori (i.e., not based on sensory experience) "reasoning" or "demonstration" which could ever lead to such knowledge. The only judgments which we can know to be true by reason alone are those Hume calls "judgments of relations of ideas" (analytic judgments). [Recall that the test for whether any statement expresses a relation of ideas is to try to deny it. If its denial is self-contradictory (logically inconsistent), then it is a judgment of relations of ideas. A judgment such as "all triangles are three sided" expresses a relation of ideas because one cannot consistently think of something which is a triangle and not three sided. Thus we can know by a priori reasoning that this judgment is true.] But there is nothing logically inconsistent in thinking of the occurrence of C and the failure of E to occur. For example, I can think of fire without thinking of heat; it is possible to imagine an idea of a "cold fire." While I have never experienced such a thing and I do in fact associate the impression of a fire with the impression of heat, there is nothing about the idea of "fire" that requires it be connected with "heat." Therefore, no judgments which express causal principles could be reached on the basis of a priori reasoning, and so any such judgments must be based on sensory experience.

Thus we are naturally led to ask, what is there in experience which leads to the belief in any causal principle? To say a causal principle is not a judgment of relations of ideas is equivalent to saying it must be a judgment of matters of fact. Hume essentially asks how do we come to believe that any given C and E are causally connected. Since a priori reasoning has been ruled out, it must lie in something we experience. But all we experience is the impression of C followed by the impression of E; we do not have any experience of the alleged "necessary connection" between them; yet this is part of the complex idea involved in thinking "C causes E."

In order to explain how we arrive at the belief that two types of events are causally connected, Hume observes that on the experience of a single C-type impression being followed by an E-type impression, we are not likely to conclude that the two are necessarily connected. However after the repeated experience of the two types of impressions being "constantly conjoined in time" (one following the other), we come to believe that the one, C, causes the other, E.

But, Hume asks, what is there is the experience of the same occurrence repeatedly that was not in the experience of a single occasion where E followed C? Hume answers it can only be that after repeated experience of C being followed by E, we come to associate the ideas, such that when we think of C occurring we come to expect E to occur. (When we think of having and impression of fire, we expect to have an impression of heat.) Hume calls such an impression of expectation formed by repeated association of C and E a "habit" or "custom" of the mind. He argues that this is the only possible impression from which one can derive the idea of necessary connection which forms part of the complex idea of causation which is present in judgments expressing causal principles of the form "C causes E."

As a consequence of this analysis of the idea of causality Hume concludes that the judgment that "C causes E" is derived from the impression of a "habit" of the mind in expecting E when C happens implies that the basis of our belief in a causal principle is "subjective," or in other words determined by the way we think, rather than "objective," determined by the nature of C and E. We may mistakenly hold that knowledge of a causal principle is based on an objective "power" ("causal efficacy") to produce the effect imagined to be in the cause, but we can have no idea of such a power because we have no corresponding impression.

What does this conclusion imply about our knowledge of the truth of any causal principle? A causal principle maintains that the cause and effect are necessarily connected, which means that whenever C happens, E must follow, or when E happens, C must have preceded it. However, our belief in such a principle is based on experience of repeated cases of C being followed by E and the "habit" of expecting this pattern to continue into the future. But this basis cannot justify the truth of the conclusion that C will always be followed by E, for the habit of expectation is purely subjective, and, since a causal principle cannot be known as a relation of ideas, no experience of the past conjunction of the two in time can ever establish that they will continue to be so conjoined in the future.

But, a critic of Hume might object, why can't we infer from past instances in which C was followed by E to the conclusion that C will always be followed by E? Hume refers to such an inference as "experimental or moral reasoning"; today we would call it an "inductive inference" because it reasons from particular premises (past cases of C being followed by E) to a universal conclusion that this connection always holds true. No such inference can ever establish its conclusion to follow with certainty from its premises. Since knowledge requires certainty and no inference to a causal principle can ever be certain, it follows that knowledge of causal principles is impossible. All we can hope for is a possibly fallible belief based on our habit of expecting experienced connections of C and E in the past to be continued into the future.

One might consider trying to turn the inference to a causal principle into a deductive inference. Such an inference would look like the following argument:

In the past C has always been followed by E.
The future will resemble the past.
Therefore, in the future C will always be followed by E.
[or, in other words, C is always followed by E; or C and E are "necessarily connected"]

The first premise can of course be known by experience. But how could we ever know the second premise? This statement, "The future will resemble the past." is known as the "principle of the uniformity of nature," but it is futile to try to appeal to it to try to prove a causal principle is true, because we can never know the principle of the uniformity of nature to be true. To see why, we need only repeat the same strategy Hume has used on causal principles. It can't be known by a priori reasoning because its denial is logically possible (it's not a judgement of relations of ideas). It can't be known by experience because we have no experience of the future. So just like any specific causal principle, the principle of uniformity of nature is simply a belief based on the habit of expecting the future to resemble the past because in the past what was then the future, when it became the present, turned out to resemble the past. But that, of course, is no grounds for certainty concerning what is still the future. We can conceive of the possibility that the course of nature could change. Thus there is no way to prove, either by experience or by reason, that the course of nature can't change, because having no experience of the future, we cannot establish it on the basis of experience, and since its denial is not self contradictory, we cannot establish it be demonstrative reasoning. So, in short, the principle of the uniformity of nature cannot be known.

How this analysis of causality lead to skepticism:

Why does the fact that no causal principle can be known lead to skepticism? Hume has argued that any knowledge of the world exterior to our mind (i.e. which "goes beyond" the testimony of our sense and the contents of our memory) requires an inference from what we know immediately, our impressions and ideas, to the alleged cause of those impressions in the external world. But what would it require to be able to give any meaning to the sort of causal principle which would be necessary to support such an inference? We would have to have experience of both C and E conjoined in time. But in this case we can never have any experience of the "C" preceding the "E," because it lies in the "external world" outside our mind, and all we experience is our impressions and ideas. So, having no impression of the presumed cause, we cannot ever formulate a causal principle which would connect this presumed cause to the impressions as its effect. Hume is not merely saying we cannot know what it is that causes our impressions, but we could possibly believe that they were caused, for example by material substances, as does the materialist, or by God's ideas, as Berkeley has argued He is making the much stronger claim that we cannot even give any meaning to the notion of a cause of our impressions lying "outside" the mind, because, by his empiricism, we can only think of that of which we can have experience. But the only things we can experience are impressions and the ideas which copy them, not some presumed "cause" of these impressions. In short we cannot ever infer from our impressions to anything at all which causes them, if indeed there even is such a cause.

Consequently, if we are indeed restricted from ever making any justified inference from our impressions and ideas to anything external, what can we know? Hume allows two possibilities: analytic knowledge of judgements of relations of ideas, but it is uninformative about the world or "reality" (so metaphysics cannot be analytic), or knowledge which is restricted to our impressions and ideas. One might mistakenly suppose that the latter option would lead Hume to a metaphysical idealism like Berkeley's and make the positive assertion that reality is simply impressions and ideas in minds. But Hume will not take this path either, for Berkeley's notion of "spirit" or "mind" as that "in" which perceptions exist is an idea for which no corresponding impression can be found. Hume argues we simply have impressions, we do not have any impression of the "mind" or "spirit" having the impressions. Just as Berkeley had shown that on empiricists' principles, one cannot have any idea of "matter" or "material substance" as some non-thinking "substance" which has the alleged primary properties, so we cannot have any notion of "mind" or "thinking substance" as that which has impressions and ideas. Thus Hume stands pat with skepticism and asserts nothing at all about the character of any reality that might (or might not) exist "external" or "beyond" our impressions and ideas. For Hume "reality" simply is impressions and ideas.