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The Problems of Philosophy is an introduction to the discipline of philosophy, written during a Cambridge lectureship that Russell held in 1912. In it, Russell asks the fundamental question, “Is there any knowledge in the world which is so certain that no reasonable man could doubt it?” Russell sketches out the metaphysical and epistemological views he held at the time, views that would develop and change over the rest of his career.

Russell begins by exploring the twin concepts of appearance and reality. Empiricists like Russell believe that all knowledge is ultimately derived from our sensory perceptions of the world around us. Individual perception, however, is easily affected and prone to error. If three people—one who’s had three martinis, one with a heavy fever, and one who’s color-blind—look at the same table, chances are they’ll each see the same object somewhat differently. Submerge the same table underwater, or set it behind a wavy pane of glass, and once again the table will look different. There is, then, a distinction to be made between appearance and reality. If perception is so variable, what can it actually tell us about the stable, real object we assume lies behind it?

Russell coined the term “sense-data” in his attempt to discern the relationship between appearance and reality. Sense-data are the particular things we perceive during the act of sensation. When you walk into a café, the smell of the coffee, the redness of the awning, and the heat from the radiator are all examples of sense-data. Sense-data are the mental images (visual as well as auditory, olfactory, tactile, and gustatory) we receive from a given object in the physical world. As we can see from the table example, the same object can produce variable sense-data. Sense-data are related to the physical objects they represent, but the exact nature of this relationship is unclear. The skeptical argument contends that sense-data tell us nothing about the reality of the object. Russell had a commonsense take on the matter: while he understood the skeptical arguments, he found no reason to believe them. A hundred different viewers may have a thousand different kinds of sense-data for a given table, yet each agrees that they are looking at the same table. This consistency suggests, to Russell, that we must at least believe in the existence of a single, particular, real table. To this “instinctive” belief, Russell also adds the hypothesis that physical objects cause the sense-data we receive and therefore correspond to them in some significant way.

During the act of sensation (i.e., the exercising of our five senses), we receive and process the sense-data produced by physical objects in our vicinity. The knowledge we gain during this process Russell calls “perceptual knowledge”—knowledge gained through experience. In contrast, Russell believes we are also in possession of certain kinds of a priori knowledge. These include the self-evident rules of logic, most important, and those of mathematics. Perceptual knowledge (the knowledge of things) and a priori knowledge (the knowledge of truths) work in concert: the first gives us empirical data, and the second tells us how to process that data.

Russell further divides human knowledge into knowledge by acquaintance and knowledge by description. To be acquainted with something is to be directly and immediately aware of it, without the action of an intermediary. When you sit on a red plastic chair, you become acquainted with lots of sense-data associated with that chair. You know its redness, its smoothness, its coolness, and its hardness. But to know that this thing is called a “chair” and that it’s often found in the company of other “chairs” and something called a “table” requires more than just direct, immediate acquaintance with the physical object. To know all that requires us to make inferences, based on our general knowledge of facts and on our acquaintance with other similar objects. This kind of knowledge is derivative, and Russell terms it “knowledge by description.” For instance, most of us know only by description that Everest is the tallest mountain in the world. Few of us have actually been there, so we have to rely on the testimony of others to “know” that fact. Indeed, to truly be acquainted with the fact of Everest’s superior height, one would have to visit and measure all the mountains in the world. It’s probably safe to say, then, that no one is truly acquainted with that particular piece of knowledge.

Just as we can know objects either immediately or derivatively, we can also know truths immediately or derivatively. Russell defines immediate knowledge of truths as intuitive truths. These are concepts that, to Russell, are so clearly self-evident that we just know they must be true. “1 + 1 = 2” is an example of such a self-evident truth. Derivative knowledge of truths involves deduction and inference from immediate, self-evident truths.

All knowledge is, in Russell’s view, built on acquaintance. Without knowledge by description, however, we would never pass beyond the limits of our own individual experience. Thus, just like perceptual and a priori knowledge, knowledge by acquaintance and knowledge by description work together to create a totality of human knowledge.


The Problems of Philosophy represents Russell’s first major attempt at mapping out a theory of epistemology, or a theory of the nature of human knowledge. Russell’s attempt to discern what kinds of knowledge, if any, could be considered reasonably certain is similar to the goal of Principia Mathematica, which is to find an undeniable reason for believing in the supposed truths of mathematics. Both branches of Russell’s work—the mathematical and the more traditionally philosophical—have at their heart Russell’s steadfast devotion to rigorous analysis and his reluctance to accept any proposition (no matter how obvious or commonsense seeming) without a concrete, logical reason for doing so.

Beginning with this work and continuing through Our Knowledge of the External World and beyond, Russell sought to describe the relationship between knowledge, perception, and physics (the study of the material, physical world). Fundamental to Russell’s theories was a belief that the physical world does, in fact, exist. Almost two decades earlier, Russell had rejected idealism—the theory that reality is not physical but exists only in the mind—in favor of realism, the belief that objects exist independently of our perception or experience. The theories of epistemology described in Problems of Philosophy fit squarely within the British empiricist tradition, in that they claim that the data gained from personal, immediate experience is the starting point of all human knowledge. In Russell’s system, data gained from personal, immediate experience are termed “knowledge by acquaintance.”

According to Russell, any proposition we know “by description” must be wholly made up of things we know by acquaintance. If we assume this, then there are some consequences for what, exactly, it is possible to know by description. Suppose you make a proposition about Julius Caesar: you say, for example, “Julius Caesar launched the first Roman invasion of Britain.” You are not actually acquainted with Julius Caesar himself, since you have no direct, immediate experience of the man. What you hold in your mind is a description of him. You may know of him as “the founder of the Roman Empire,” for example, or “the man assassinated on the Ides of March,” or “the subject of the marble bust in my local library.” Thus, when you say, “Julius Caesar launched the first Roman invasion of Britain,” you’re not really asserting something about the real Julius Caesar—you can’t be, as you have no direct knowledge of him. Instead, you’re asserting something about the collection of facts and ideas about Caesar with which you are acquainted. No matter how many facts we may learn about Caesar, we can still only know him by description. We can never reach a point where we directly know him by acquaintance. The general thrust of this argument foreshadows Russell’s work in logical atomism, which argues that statements can be broken down into a series of constituent assumptions. The argument is also tied to Russell’s Theory of Descriptions, which explains how definite descriptions—phrases like that cat, Bill Cosby, or my mother, which refer to specific, particular objects—are just shorthand for a series of logical claims. Similarly, when we use the phrase Julius Caesar, we’re using the name to refer not to the man himself but to a series of facts and descriptions we have learned about him.

The Problems of Philosophy was meant to be an introduction to the field, and as such, Russell’s arguments aren’t as thorough as we might expect from the founder of analytic philosophy. He often errs on the side of “illustrating” his points rather than meticulously mapping them out. While the book makes strong appeals to common sense, there are still elements that have greatly troubled critics. One such problem lies with Russell’s notion of intuitive knowledge. Russell never satisfactorily explains what, exactly, makes a truth self-evident, and he does not provide sufficient examples of these intuitive, immediate truths. Russell also provides no plan for distinguishing between two apparently self-evident truths that nevertheless contradict each other.

The concept of sense-data, as set out by Russell, has also proved problematic. Russell takes it as a given that sense-data are the building blocks of perception. We look at a table and we sense its brownness, its hardness, and its rectangularity. From these sense-data, we construct our idea of the table. Other philosophers argue that, upon seeing a table, we are immediately aware of the object as a table, and it is only later, when we stop to concentrate on what we see, that we consciously notice the object’s color, its texture, or its shape. According to these thinkers, sense-data as defined by Russell cannot be the most primitive, direct element of experience because it requires too much conscious effort to be aware of them.

Finally, a major issue in Problems of Philosophy lies in the fact that, to Russell, all knowledge is built on knowledge by acquaintance, or the things we know through direct, personal experience. Russell accepts a fundamentally Cartesian point of view, which means he accepts that the proper foundation for philosophical inquiry is individual consciousness and perspective. But how can a theory of knowledge be built on private experiences if this theory is supposed to apply to all beings? This problem (among others) bothered Russell, and in his next major epistemological work, Our Knowledge of the External World, he begins to push his inquiry into the public sphere.

Popular pages: Selected Works of Bertrand Russell