Let me introduce "An Enquiry Concerning Human Understanding", a philosophical treatise written by David Hume.
David Hume is one of the most prominent empiricist philosophers of the enlightenment era. Contemporary readers, who have seriously investigated his writings, will readily come to the conclusion that his reductionist and experience-oriented worldview vividly parallels that of today's natural sciences. As a result, it is not hard for us to feel the invaluable contribution of his philosophy to the development of the modern mind, as well as its emphasis on the usage of scientific methods in discovering the secrets of our universe.
"An Enquiry Concerning Human Understanding" is one of Hume's most popular treatises which expounds many aspects of his empirical worldview. It is not as widely known as his previous book called "A Treatise of Human Nature", but it is nevertheless a great primer for those who are interested in Hume's philosophy because it is written in a relatively concise manner (I have to say "relatively" because it is still pretty long).
Hume begins his treatise by categorizing his elements of reason, such as "perceptions" (which can be subdivided into two classes called "impressions" and "ideas") and "objects of reason" (which can be subdivided into two classes called "relations of ideas" and "matters of fact"). The former refers to the basic building blocks (i.e. atoms) of our mental phenomena, and the latter refers to the ways in which these building blocks can combine with one another to form more complex meanings.
By means of categorization, Hume establishes a firm ground of terminologies which allows him to deliver his metaphysical concepts to the reader without causing too much confusion. One may claim that the way he draws artificial boundaries between seemingly intertwined concepts (e.g. between "impressions" and "ideas") is a bit too contrieved, yet I believe that it is a necessary evil which had to be maintained in order for Hume to give himself some solid pieces of intellectual toy to play with.
One of Hume's most widely discussed topics in this treatise is the issue of causality. According to his empirical faculty of reasoning, we can never directly sense the force of "necessary connexion" between a cause and its effect. All we can say from our experiences is that one type of events tend to occur in close proximity with another type of events, and that a high frequency of such correlative occurrences is the only clue which suggests us that these two types of events are causally related to each other (i.e. one is the cause and the other is the effect).
Hence, the very notion of causality itself, according to Hume as well as other highly empirical thinkers such as Bertrand Russell, may as well just be a manmade concept and not a fundamental part of reality. For example, when we observe that event B always immediately follows event A, does it mean that event A must be "causing" event B to occur? Wouldn't it also be plausible to suppose that A and B are just two predetermined parts of one underlying event?
To be honest, most of the key lessons of this treatise are contained in the first four sections, which occupy roughly the first 25% of the volume. The rest of the book is, in my opinion, a bit verbose and spends rather too many words to explain the same old concept over and over again, and it feels more of Hume's personal rant against his philosophical opponents than an impersonal effort to unravel his line of logic. My impression is that he dedicated a huge portion of his book to express his own sentiments in regard to how his philosophy was unjustly dismissed and jeered at by backward thinkers.
However, I also would like to say that not all of his personal remarks deserve to be skimmed so lightly as though they are mere byproducts of his momentary indignation. During the very first section of his treatise (namely, "Of the Different Species of Philosophy"), Hume expresses his frustration with the fact that most people prefer "easy philosophy" which is inspired by emotional appeals and a strong sense of entertainment (e.g. political propaganda), over a more objective and truth-seeking kind of philosophy which, despite its sincere endeavors to touch the very essence of our philosophical problems, fails to win the heart of the mass because it does not sugarcoat itself with worldly passions.
And I must confess that this first section of the book really touched my heart - far more than I would expect from a philosophical treatise. It made me understand the sheer sense of frustration which Hume, after publishing "A Treatise of Human Nature", must have felt when he realized that his ambitious work was almost completely ignored by the public, and that his intellect had no place in this world.