"The Greatest Problem In Philosophy" may be a bit misleading. Allow me to clarify. The issue I wish to articulate in this piece is peculiar to that branch of philosophy known as "epistemology," or theory of knowledge and justification.

David Hume's "problem of induction" is — in my opinion — the greatest epistemic puzzle ever invoked. It calls into question the rationality of human beings and likens their cognitive faculties to that of inferior animals. The "problem of induction" may not have a very appealing name, but if you take the time to understand the argument and its implications, you will find it says something profound about human nature.

First off, what is induction? Induction is the logical process of generalizing on the basis of individual instances. This is contrasted with deduction which predicts individual instances on the basis of general laws or rules. Induction claims that since we've observed individual instances of the sun rising in the East and setting in the West every day for as long as we can remember, the sun will continue to do so tomorrow and for the next 100, 1,000 and million years. Deduction says that 1+2=3 and that all bachelors are unmarried men because of the meaning of the constituent elements involved.

Notice that deduction rests on necessity. It is necessary that 1+2=3 because of what we mean by the individual terms involved. It is necessary that all bachelors are unmarried men because of the meaning of bachelor. It is impossible for a bachelor to be married because a bachelor, by definition, is unmarried.

Induction, however, involves no necessity. Even though we have observed the sun rising every day in the past, it does not guarantee that the sun will rise tomorrow. Whenever you've eaten a peanut butter and jelly sandwich in the past and were nourished by it, does not mean it will continue to nourish you the next time. Even though you've opened a history book and read about Napolean's defeat at the Battle of Waterloo in 1815, does not mean you have been granted with absolute knowledge. Historical facts change all the time, though there is a very small probability historians are wrong about the date of Napoleon's defeat.

Hume has a problem with induction. He believes humans can't justify any conclusion they reach by using induction, which is based on past experience. I can't justify, in any absolute sense, that the sun will rise tomorrow, that I will not fall through the floor when I get out of bed in the morning, or that Napoleon was defeated in 1815. Perhaps a meteor will hit the Earth and blast it out of orbit with the Sun. Perhaps I was unaware of the termite problem underneath the infrastructure of my bedroom floor. Perhaps the man who recorded Napoleon's defeat at Waterloo was mentally insane and historians have been relying on faulty evidence. There are a number of reasons why we might be wrong about our understanding of the external world, and the fact that past experience does nothing to justify our beliefs only adds to the problem.

Hume is led to conclude that humans, much like non-human animals, come to learn things through habit, not reason. Though this may seem an unsettling answer at first, it really is the only one that follows from the premises.

There is much more to the argument but for the sake of simplicity, I will stop here. Hume's reasoning forces anyone who recognizes its logical appeal to fall into skepticism about all matters concerning the external world, if even a little. He presented his argument in the 18th century and it has yet to come face-to-face with a satisfactory rebuttal.