Russell wasn’t completely satisfied with his theories as laid out in The Problems of Philosophy and continued his work on knowledge and perception over the next several decades. One of his important contributions to the field is Our Knowledge of the External World, a collection of lectures published in 1914. In this book, Russell continues to struggle with the implications of his Cartesian assumption—that private experience is the proper place to begin philosophical inquiry. Russell doesn’t reject that notion wholeheartedly, but he is very aware of the difficulties it raises. He is aware that crossing over from the “private space” of personal experience and sensation into the “public space” of science and the physical world is a difficult leap to logically justify.

Following from Problems, one thrust of Our Knowledge of the External World concerns itself with the relation of perception and physics. Russell continues to ask, can we come to know things about the physical world through the actions of our senses? And if so, how do we know these things? In both this book and in an article titled “The Relation of Sense-Data to Physics” (also from 1914), Russell returns to the concept of sense-data, which he described earlier in Problems. Sense-data are the characteristics and qualities we primitively, immediately sense about physical objects: color, shape, texture, temperature, and so forth. Previously, Russell had argued that sense-data were the functions of physical objects. That is to say, physical objects cause sense-data, which we then perceive when we exercise our five senses. A cat exists in the real, physical world, and from that cat we sense warmth, softness, grayness. The problem with this theory, however, is that we are only acquainted with the sense-data of warmth, softness, and grayness—we infer that a cat is causing these things, but we cannot know for sure that such a thing exists.

Russell argued that the most fundamental principle of scientific reasoning is that, “whenever possible, logical constructions are to be substituted for inferred entities.” With this in mind, Russell executes a kind of flip. Instead of saying that physical objects create sense-data, he turns the matter around and argues that sense-data construct the physical object. Sense-data don’t just testify to the existence of physical objects, they essentially create the physical world. They do so in tandem with what Russell calls sensibilia. Sensibilia are “unsensed sense-data”—that is, how an object appears when no one is perceiving it at a given moment. This accounts for an object’s continued existence in the absence of perceivers. An important consequence of this theory is the notion that sense-data are not simply images held in the mind but are instead the actual building blocks of physics. Thus, sense-data inhabit the public space of science as well as the private space of experience.

A second major component of Our Knowledge of the External World is Russell’s presentation of logical atomism. This is his most important discussion of the theory, although logical atomism is also covered in several of Russell’s other publications.

In terms of physics, atomism is the theory that all matter in the universe is made up of tiny, finite particles that cannot be broken down into anything smaller. The cup sitting in front of me looks like a solid, whole object but is in reality a swarm of individual whizzing atoms. The idea of a “cup” is a convenient fiction that obscures the reality of the atomic situation. Even if we know that the cup is made up of millions of tiny particles and a lot of empty space, it’s easier to simply accept that it is a solid cup and move on with our day. The physicist doesn’t have that luxury, of course, and neither does the dedicated philosopher. Logical atomism proposes that the theory of atomism extends to other areas beyond matter—most important for Russell, to the fields of language and of knowledge. In terms of linguistics, Russell showed how seemingly ordinary statements could be analyzed to reveal a string of simpler, more elemental assumptions, which could then individually be judged either true or false (see Logical Atomism, p. 373 ).

In Our Knowledge of the External World, Russell shows how logical atomism applies to the question of knowledge and the physical world. Russell combines logical atomism with empiricism. He proposes that the “atoms” of our knowledge are the sense-data with which we are directly acquainted. Our immediate sensory experience is the only knowledge we can genuinely claim, and all other knowledge is inferred or deduced from it. Consider the cat described earlier. The cat’s sense-data—warmth, softness, grayness—are the “atoms” of our knowledge about that object. The cat that we infer from that sense-data is only a “logical fiction.” Objects are nothing more than systems of sense-data. Here, we can see a seemingly logical basis for Russell’s assertion that sense-data create rather than simply testify to the existence of physical objects.


After advancing his theory of how sense-data create the physical world, Russell abandoned it in future works and reverted to the notion that physical objects could legitimately be inferred from sensory experience. This was partially due to scientific advances in physics and human physiology, which were asserting that perception is, in fact, caused by the effects of the physical world on our sense organs. Russell also sensed other difficulties in his theories on physics and perception. For one, his notion of sensibilia is difficult to establish: what, exactly, does it mean for there to be “unsensed sense-data?" How can there be an element of perception when no one is present to do the perceiving? Russell was unable to adequately describe his system of private and public spaces or to explain how sense-data and sensibilia interact with that system. In later work like The Analysis of the Mind (1921), Russell stops treating sense-data and the act of sensation as separate entities. He does, however, maintain the classic empiricist position that physical objects are not directly knowable; only their sensory effects (what he took to calling percepts, as opposed to sense-data) are available to us. Eventually, Russell abandoned his inquiries into the relationship of matter and perception, though he continued to work in other areas of epistemology.

Critics also found flaws with Russell’s presentation of logical atomism, which was often sketchy in areas. His former student Ludwig Wittgenstein, for example, disagreed with the idea that logical atomism should be tied to empiricism, and in his Tractatus Logico-Philosophicus Wittgenstein treated logical atomism as a purely formal theory. Critics have also taken issue with how Russell defines the “simple” elements of the world’s structure. Russell says that the basic atomic facts of the world with which we can become acquainted are of two kinds: particulars and universals. Particulars are individual instances of sense-data, whereas universals are concepts that apply to many objects. These include qualities (like redness, softness, heaviness) and temporal and spatial relations (before, on top of, next to). Russell contends that particulars and universals are atomic “simples”—that is, they are finite and individual and cannot be analyzed or broken down further. However, it is difficult to see how this definition could apply to a universal like “redness.” The very concept of redness requires us to compare different objects and classify them as similar; this being so, it is impossible for “redness” to be an independent entity.

Other critics have refuted Russell’s logical atomism in myriad ways, many of which are too complex to cover here. Although Russell’s presentation of logical atomism may have proved untenable, it remains an important moment in the history of philosophy. Like the Principia Mathematica—another of Russell’s projects that eventually proved largely unjustifiable—Russell’s defense of logical atomism is spurred by an intense interest in justification. Russell’s work throughout his career can be characterized by an extreme reluctance to believe any proposition without a firm, sound reason to do so. Logical atomism was one of the earliest manifestations of analytic philosophy, which (in its most general sense) holds that philosophy should aspire to the precision and exactitude of the sciences. As Russell was one of the founders of analytic philosophy, his work inspired philosophers to rigorously examine their own assumptions and to avoid taking seemingly self-evident truths for granted. It is this dedication to constant, consistent analysis that is Russell’s greatest legacy to philosophy.

Popular pages: Selected Works of Bertrand Russell