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.