Hume's extreme skepticism about any empirical, observed knowledge of the external world is based on a very misguided and extreme empiricism. Although he allows for knowledge of certain simple mathematical and logical truths, anything beyond that is not directly supported by experience, and thus condemned unreasonable. To reconstruct his argument briefly: when interacting with the external world, we can be sure of our impressions of the sensations from the external input. However, we can never be certain about anything regarding these impressions other than that we experience them. Thus, making any sort of rational statement about these impressions goes beyond our experience. We are so uncertain about these impressions that we cannot be even sure that impressions we have received in times prior following a certain pattern will continue to do so. This is what some philosophers have called the Principle of the Uniformity of Nature (PUN). It is because of PUN that we believe something causes another thing. This principle is in fact an inferred hypothesis (induction) that has appeared to work in the past; but because we can’t truly experience this uniformity—only the perceived effects of it were it true—there is no rational way we can justify our own belief in its (PUN’s) existence. Moreover, because most, if not all observational (i.e. scientific) knowledge is based on this assumption, we have no more reason to believe in scientific knowledge than any other method of obtaining knowledge—methods a scientific person may consider to be complete hogwash (i.e. tarot, soothsaying, etc).
Obviously Hume goes wrong somewhere in this argument. Scientific knowledge does, more often than not, make accurate predictions about the future. They predict astrological phenomena all the time, for one. We also rely on technologies that have been enabled for use by scientific and engineering advances only producible by the application of scientific formulas that have at least at some point or another been tested with the rigor of experience. But finding these types of examples would not persuade anyone just yet, we must somehow prove the validity of the Principle of the Uniformity of Nature.
To be quite honest I’ve always been inclined to just toss aside skepticism of this nature because it seems so set against the intuitive outlook of the world that it can’t possibly be right. But let us give the skeptic a fighting chance. First, the Principle of the Uniformity of Nature is the assumption that underlies any notion of cause and effect. No principle of the uniformity of nature and no induction is possible. Inductions of all shapes and kinds would be impossible. If the clouds were really dark outside, the temperature dropped various degrees, and you heard a couple of thunderclaps, you would have no more reason to believe that there was going to be a nasty thunderstorm than if you were to believe that a sandstorm were to appear where you stood. Or that you were going to spontaneously combust. This is ludicrous!
We experience the PUN because we continue to see patterns emerging in significant ways in our everyday lives; we are actually more reasonable to believe in it than not. If you were to eat, among other things, tomato one day and you break out in hives, you realize that something you ate today might have brought those hives. After some self experimentation, or a visit to an allergist, you realize that eating tomatoes will be directly followed by those hives, and assuming you don’t like hives, you will discontinue your ingestion of tomatoes. Being a reasonable person, in my opinion, is at least in part recognizing that these patterns do emerge, identifying the relevant patterns in your life, and the acting accordingly. These patterns are experienced if only we expand on our notion from the Humean, mostly barren idea that we can only experience sense impressions, to a more broad conception of experience as the interaction between the senses and our own intellect to analyze it.
The main problem justifying this PUN philosophically is that it is an induction. Inductions are, by definition, logical inferences that will never be 100% certain. They are the proposal of a general rule or probability based on the experience of one thing following another significant amount of times. Inductions themselves are not deductively provable. Deductions are the other way around, there is a rule, and we are used to apply that rule in order to get the output. Deductions actually take less thought, because as long as you understand the rule, you can never misuse it. Inductions are a bit more synthetic, because it requires one to recognize patterns. But the induction itself, as a mechanism of pattern-recognition, is not itself applicable to any deduction, and it is thus deductively invalid. But this is okay! It doesn’t have to be!
The practice of creating and using inductions is valid because it works. Inductions have worked, do work, and there is no reason to believe that they will stop working any time soon. We have a great, vast amount of evidence that inductions have worked. Every time that we set foot in a car we are actually making an induction—that it will work just like the last time, that our seatbelt will actually work according to the standard safety regulations—and we are betting our lives on it. I guess this might be a pragmatic justification of induction and, by extension, the PUN.
This is what happens when empiricism is taken to the extreme. In the world of Hume, where no induction is rationally valid or justifiable, we are blind rats living in an ever evolving maze without any cheese. Sure, we can never be 100% sure of our inductions or any observational knowledge, we’d be bigger fools to think we have no rational reasons to believe them.
