David Hume wrote An Enquiry Concerning Human Understanding in 1748, eight years after the less than successful reception of his book A Treatise of Human Nature. Enquiry reworked some main points of Treatise (Book 1) that would prove to be highly influential in philosophy, especially in epistemology. In §4 and §5 of Enquiry Hume formulates an argument that would come to be known as “The Problem of Induction”. The problem of induction raises doubts relating to the validity of empirical claims such as those made by using the scientific method. I will provide an overview of his argument along with implications of his conclusions related to inductive reasoning. I will then discuss Hume’s claim that justified belief is not possible when the assumption is made that the future will resemble the past. Lastly, I will argue that Hume is distinguishing between two different types of knowledge that does not exclude inductive reasoning because of the difference between philosophical skepticism and ordinary life.
Hume begins §4 by distinguishing between "relations of ideas" and "matters of fact”. Relations of ideas are propositions that do not require experience or are a priori. Relations of ideas are also logically true and as Hume stated, “… discoverable by mere operation of thought without dependence on what is anywhere existent in the universe”. Denial of such a proposition would lead to a contradiction just as denial of the statement “all bachelors are unmarried” would be contradictory. Hume argues that matters of fact deal with experience or are learned a posteriori and cannot be disproved by an appeal to reason. If I make a matter of fact statement such as “it is raining outside” one cannot disprove my statement by reasoning alone but may do so by looking out the window and observing the sun shining. Hume suggests that we know matters of fact about unobserved things through a process of cause and effect. I have knowledge that my essay is due next Wednesday and that the sun will rise tomorrow by referencing a syllabus or inferring from past experience. But how do we know of the principle of cause and effect? He suggests that this knowledge cannot be a priori but “… arises entirely from experience, when we find that any particular objects are constantly conjoined with each other”. He also points out that matters of fact allow for the possibility of contradiction. For example, it is not inconceivable that the sun will implode and not rise tomorrow (matter of fact) unlike the concept that tomorrow a triangle will have four sides (relations of ideas).
Hume starts Part II of §4 by reminding us that the nature of our reasoning concerning matters of fact relates to cause and effect while the foundation for that reasoning rests upon experience. It is here that he asks upon what do we rest the foundation of all conclusions from experience. He distinguishes between two types of propositions:
1. “I have found that such an object has always been attended with such an effect,”
2. “I foresee, that other objects, which are in appearance, similar, will be attended with similar effects”
The first proposition refers to present and past events while the second refers to future events based on past events. Hume argues that we cannot know that the future will resemble the past by means of demonstrative reasoning of matters of fact since there is no contradiction in suggesting that the future will not resemble the past. If all our predictions about the future are based on this principle and that principle is derived from past experience, we cannot know that it will remain true in the future except by assuming that principle from the outset. Hume suggests that we infer to the future similarities from the past but that there is no form of reasoning that can justify these inferences with absolute certainty.
At the end of §4, Hume confesses that he may simply have failed to identify an argument that could give a rational foundation for inferences to the future. He claims, however, that we learn to infer matters of fact not through reasoning but through the conditioning of custom. For example, a child or “brute beast” knows from experience that a flame will burn. Hume does not suggest that we abandon experiential learning or inference to the future from past events but only that, from the point of view of a philosopher, there is not a reasonable, discernible argument that can justify such an inference. In this respect, he is only a philosophical skeptic not a skeptic as an ordinary person.
In §5 Hume attempts to show how and why we necessarily make inferences from the past to the future and other inductive connections even though he showed in the prior section that true justification is not possible. At the beginning of §5, he expands on the differences between the ordinary life and that of “academic or skeptical philosophy”. He warns the reader that passionate philosophy, like religious thought, runs the risk of being inclined to “… reason us out of virtue, and of social enjoyment.” He claims that nature always wins out against abstract reasoning but argues that there is value in further enquiry to discover the principles behind natural processes. In this case, he is referring to the type of reasoning required to justify assumptions made by inferring past events onto the future. The main point in the beginning of this section is to remind us that he is not advocating deep skepticism relating to induction in everyday life but has the right to do so in a philosophical sense.
Hume then sets up a thought experiment to understand the process of cause and effect by asking us to imagine someone that has been thrust into the world with the strongest faculties of reason and reflection but without experience. He claims that this person would see a succession of events but would not be able to discover what would come next because he would not yet have a sense of cause and effect or custom. The world would not make sense because without custom, reasoning concerning matters of fact could not extend beyond memory and sense experience. We could not speculate nor act if custom did not give us the ability to see certain actions as having certain consequences. According to Hume, “Custom, then, is the great guide of human life. It is that principle alone, which renders our experience useful to us, and makes us expect, for the future, a similar train of events with those which have appeared in the past.”
Hume argues, however, that all reasoning from experience ultimately falls back upon simple impressions. What I know about current events may rest upon the impression I get from the local news channel and what I speculate about the future might be based upon impressions I am making in the present. In other words, our speculations about unobserved matters of fact rest upon a constant conjunction with our present impressions. Hume suggests that we make inferences by means of the imagination but distinguishes between fiction and belief. Fiction is the product of pure imagination while belief is a combination of imagination and a certain sentiment that we cannot control which corresponds to reality. When memory or sense impression is present to us, custom will engage belief and we will infer from that impression to what it is constantly conjoined. This force of custom functions on our beliefs and helps us make sense of the world.
Hume then reduces his argument regarding how we make inferences to three principles of connection as follows:
1. Resemblance – the likeness or absence of likeness relating to ideas or affect.
2. Contiguity – sequential or proximity to ideas or affect.
3. Causation – relates to the belief of correlation between events.
These 3 principles “… are the only bonds, that unite our thoughts together, and beget that regular train of reflection or discourse…” Custom and the three principles of connection allow us to find harmony between the course of nature and the succession of our ideas without which we could not function. He does not claim to understand the prima causa to the phenomenon but he notes that this has not stopped enquiry in other phenomena of nature. He concludes §5 by reinforcing the idea that inference is essential for human subsistence even though it is susceptible to error, sometimes slow in operation, and does not appear in the first years of infancy. Lastly, he comments that just as nature has given us use of our limbs without knowledge of the muscles, nature has also given us instinct through which the principles of inference and connection are utilized without knowledge of how this truly functions to justify belief.
The implications of Hume’s skepticism are profound. Although he allows for the contrary by appealing to the ordinary life, from a philosophical standpoint, Hume’s argument could be interpreted in 2 ways:
1. When we reason inductively we are assuming the future will resemble the past and we can never justify that assumption.
2. When we reason inductively we are blindly following a way of thinking that cannot be shown to be reliable.
Interpretation #1 is implicit in §4 Part II and is what he is generally known for as a skeptic. If we cannot reliably or rationally justify any inferences based on previous connections then everything we believe will happen must be irrational. Accordingly, the only reasoning we may trust is a priori or based on immediate sensations. Even then, impressions are subject to error and as such, may not be used as rational justification of belief. This supports both interpretations because the justification of only immediate sensation excludes inductive reasoning of any sort because connections necessarily involve temporally prior events. In that respect, we are blindly following a way of thinking that cannot be shown to be reliable. The fact that we sometimes make judgment errors or have false beliefs only supports this notion. Hume is an empiricist but strictly so because of the doubt he associates with any reasoning by connection. The implication of Humean skepticism is also profound to science because observations and connections are inherent to the development of hypotheses and models of prediction. In a nutshell, Hume supports the claim that most reasoning cannot be trusted and therefore cannot be justified. As I will show, however, Hume would not argue that we abandon science, question every single belief we possess, or claim that we are irrational by nature.
At the beginning of §5 Hume distinguishes in more detail between rationality related to the philosophical skeptic and that of an ordinary person. He does so because he realized that the knowledge and arguments of the time justified his argument against inductive epistemic certainty. He also realized that acceptance of his argument in day-to-day life was not only unrealistic but infeasible as well. He is correct to claim that epistemic certainty is out of reach due to the nature of our senses and reasoning; however, our nature, senses, and reasoning seem to do a good job of keeping us alive and moving along the path of progress. Humean skepticism would grind life to a halt if we fully accepted that every connection and association might be faulty. What would prevent the child from repeatedly putting her hand into a flame other than inductive reasoning? The real value of the argument against inductive reasoning lies in epistemology and science because it is important to remember that our reasoning may be faulty and that our belief may not be fact. The evolution debate is an example of why this important. Observation and connections across multiple fields of study strongly support the theory of evolution yet many skeptics dismiss it as “only a theory” and not fact. This is frustrating for those that support the theory but it only encourages further study to find support that will answer the challenges of the skeptics. It may be possible that science or philosophy will eventually provide a valid argument for inductive reasoning that defeats Hume’s claim. Until then, Humean skepticism demands constant academic questioning that can only benefit those that walk through everyday life with somewhat faulty reasoning.
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