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.