David Hume was born in Edinburgh, Scotland, to a proud family of moderate means. He was educated in Britain, and moved to France at age 23 to write his first novel. He hoped the book would sell well, inspire a generation of new thought, and make him rich. It was titled A Treatise of Human Nature, and Hume had difficulty finding a publisher who would print the manuscript. As with many geniuses, Hume encountered much discouragement before his ideas were accepted. When the Treatise finally appeared in 1738, it "fell dead-born from the press" (in Hume's own words). Not only was the book a terrible seller, it failed even to achieve Hume's goal of challenging traditional thought. The religious zealots who were supposed to have been so offended by his ideas had never even heard of them. Hume eventually re-published his Treatise in the form of two Enquiries, An Enquiry Concerning the Human Understanding (1748) and An Enquiry Concerning the Principles of Morals (1751). These helped spread his ideas and fame, although it is the republished and complete Treatise that I will focus on. The system of ideas outlined therein, with an included essay specifically on Self, will be enough to explore Hume's thoughts on the subject.
Hume's theory of knowledge taught that "accurate and just reasoning...is alone able to subvert that abstruse philosophy and metaphysical jargon, which, being mixed up with popular superstition, renders it in a manner impenetrable to careless reasoners, and gives it the air of science and wisdom." In other words, the philosophies (science and wisdom) of the learned in Hume's day were abstruse speculations designed to defend superstitions that can't be justified in logical argument. Hume saw his "accurate and just reasoning" as the only means to attack the grandiose metaphysical speculations of those learned Enlightenment thinkers. His skepticism included more than those metaphysical speculations; he was supremely critical of all tradition and assumption in human reasoning.
The singular most important aspect of Hume's philosophy may be his confidence in the senses. Again and again throughout his Treatise and other works, Hume restates his skepticism of all that is not in some way connected with our senses. Even the common ideas of God, Causality and Self were rejected by Hume, who claimed to have no sense experience of any of them.
Hume first separated human perceptions into two distinct categories. The "less forcible and lively" perceptions he called thoughts or ideas. The more lively perceptions of the mind Hume labeled Impressions. The impressions are what we see, hear, feel, love or hate. Our ideas are the less lively perceptions that reflect upon our impressions. For example, if I place my hand upon a hot plate and feel the heat, that is an impression. Later, when I go over the experience in my thoughts, I may recall the heat, I may remember the burning pain, but I cannot truly experience the sensation again. My "less-lively" thought occurs only in relation to my "lively" sense data, but while I am having that thought I do not truly experience the sense data again. Perhaps the one assumption Hume did make was that all of these simple ideas (perceptions) are distinct, and independent of one another . Some have claimed this to be the one flaw in his argument, the one assumption he should have examined further; others debate whether Hume held this view at all. I will assume that he did view simple ideas as independent entities, based on my reading of his texts.
Next, Hume questioned the order and association of simple ideas. If they were, as he said, independent of one another, it should be theoretically possible for one simple idea to precede or follow another, at random order. A simple examination of human thought processes will show that this is not the case, as Hume realized. He searched for a "universal principle" among our ideas, "some bond of union among them, some associating quality, by which one idea naturally introduces another." Furthermore, Hume labeled complex ideas, the common subjects of our thought and reasoning. For example, when someone mentions an "apple", I will think of some apple I know - that is, I have an image of an apple I've previously encountered which exemplifies the concept of "apple" in my mind. This apple is a complex idea, it is a notion that I can apply to all such objects in my mind, and use to communicate with other human beings. That is, when two people talk about an apple, they both understand the idea of "apple"; yet when each person mentally visualizes "apple", each has a separate and distinct vision of an apple they have seen on a previous occasion. It may be said that the two are not really talking about the same apple at all.
To reiterate: all simple ideas are memories of simple sense perceptions. All complex ideas are combinations of simple ones. Hume argued next that a term can only have meaning if it is (1) a sense impression, (2) a copy or memory of a sense impression or (3) a combination of impressions of which it is a copy. Hume wanted to throw out everything that wasn't related in some way to a simple idea, or a combination of simple ideas in the form of a complex idea. This critical view he applied to several well-known philosophical subjects, most especially that of the Self, or personal identity .
Many philosophers contend that we are always conscious of an idea of our Self, that it exists and continues to exist. To these philosophers, Self is an innate idea, to attempt to prove it will weaken the evidence that it exists. This is a scary path of reasoning; without the Self there is nothing of which we can be certain. Hume asked a simple question "From what impression cou'd this idea of Self be deriv'd?" He knew that there was no answer, to attempt an answer results in an immediate contradiction. Hume realized that if any one impression gave rise to the concept of Self, that impression should remain the same throughout our lives; the idea of Self would have to exist according to that impression. Problem is, no sense impression is constant and invariable. Pain and pleasure, joy and despair, all exist in our lifetimes, and not at the same time. Since no sense impression remains the same, Hume concluded that the idea of Self could not be derived from any of them.
Next, Hume examined our perceptions and their relation to the Self. He considered the senses distinguishable and separable from one another; "they may exist separately, and have no need of any thing to support their existence," as written in the Treatise. If impressions are truly independent entities, how, then, do senses belong to the self? How are they connected with it?
When Hume tried to enter into that most intimate place he called myself, he always found sense perceptions or combinations of sense perceptions there. When visualizing his Self, Hume found only hot and cold, pain and pleasure, light and shadow. He could never find his Self at any time when it wasn't relating to a perception, and could never conceive of anything but the perception. Hume also pointed out that if our perceptions were removed by death, we could no longer think, feel, see, act, love or hate, we would be entirely and completely annihilated. A total lack of perceptions is all that is required to reduce a human being into a complete non-entity. Thus, the idea of Self was innately flawed and impossible; it derived from no simple impression and could not exist without such impressions.