After distinguishing two types of knowledge, knowledge of things and knowledge of truths, Russell devotes this fifth chapter to an elucidation of knowledge of things. He further distinguishes two types of knowledge of things, knowledge by acquaintance and knowledge by description. We have knowledge by acquaintance when we are directly aware of a thing, without any inference. We are immediately conscious and acquainted with a color or hardness of a table before us, our sense-data. Since acquaintance with things is logically independent from any knowledge of truths, we can be acquainted with something immediately without knowing any truth about it. I can know the color of a table "perfectly and completely when I see it" and not know any truth about the color in itself. The other type of knowledge of things is called knowledge by description. When we say we have knowledge of the table itself, a physical object, we refer to a kind of knowledge other than immediate, direct knowledge. "The physical object which causes such-and-such sense-data" is a phrase that describes the table by way of sense-data. We only have a description of the table. Knowledge by description is predicated on something with which we are acquainted, sense-data, and some knowledge of truths, like knowing that "such- and-such sense-data are caused by the physical object." Thus, knowledge by description allows us to infer knowledge about the actual world via the things that can be known to us, things with which we have direct acquaintance (our subjective sense-data).
According to this outline, knowledge by acquaintance forms the bedrock for all of our other knowledge. Sense-data is not the only instance of things with which we can be immediately acquainted. For how would we recall the past, Russell argues, if we could only know what was immediately present to our senses. Beyond sense-data, we also have "acquaintance by memory." Remembering what we were immediately aware of makes it so that we are still immediately aware of that past, perceived thing. We may therefore access many past things with the same requisite immediacy. Beyond sense-data and memories, we possess "acquaintance by introspection." When we are aware of an awareness, like in the case of hunger, "my desiring food" becomes an object of acquaintance. Introspective acquaintance is a kind of acquaintance with our own minds that may be understood as self-consciousness. However, this self-consciousness is really more like a consciousness of a feeling or a particular thought; the awareness rarely includes the explicit use of "I," which would identify the Self as a subject. Russell abandons this strand of knowledge, knowledge of the Self, as a probable but unclear dimension of acquaintance.
Russell summarizes our acquaintance with things as follows: "We have acquaintance in sensation with the data of the outer senses, and in introspection with the data of what may be called the inner sense—thoughts, feelings, desires, etc.; we have acquaintance in memory with things which have been data either of the outer senses or of the inner sense. Further, it is probable, though not certain, that we have acquaintance with Self, as that which is aware of things or has desires towards things." All these objects of acquaintance are particulars, concrete, existing things. Russell cautions that we can also have acquaintance with abstract, general ideas called universals. He addresses universals more fully later in chapter 9.
Russell allocates the rest of the chapter to explaining how the complicated theory of knowledge by description actually works. The most conspicuous things that are known to us by description are physical objects and other people's minds. We approach a case of having knowledge by description when we know "that there is an object answering to a definite description, though we are not acquainted with any such object." Russell offers several illustrations in the service of understanding knowledge by description. He claims that it is important to understand this kind of knowledge because our language uses depends so heavily on it. When we say common words or proper names, we are really relying on the meanings implicit in descriptive knowledge. The thought connoted by the use of a proper name can only really be explicitly expressed through a description or proposition.
Bismarck, or "the first Chancellor of the German Empire," is Russell's most cogent example. Imagine that there is a proposition, or statement, made about Bismarck. If Bismarck is the speaker, admitting that he has a kind of direct acquaintance with his own self, Bismarck might have voiced his name in order to make a self-referential judgment, of which his name is a constituent. In this simplest case, the "proper name has the direct use which it always wishes to have, as simply standing for a certain object, and not for a description of the object." If one of Bismarck's friends who knew him directly was the speaker of the statement, then we would say that the speaker had knowledge by description. The speaker is acquainted with sense-data which he infers corresponds with Bismarck's body. The body or physical object representing the mind is "only known as the body and the mind connected with these sense-data," which is the vital description. Since the sense-data corresponding to Bismarck change from moment to moment and with perspective, the speaker knows which various descriptions are valid.
Still more removed from direct acquaintance, imagine that someone like you or I comes along and makes a statement about Bismarck that is a description based on a "more or less vague mass of historical knowledge." We say that Bismarck was the "first Chancellor of the German Empire." In order to make a valid description applicable to the physical object, Bismarck's body, we must find a relation between some particular with which we have acquaintance and the physical object, the particular with which we wish to have an indirect acquaintance. We must make such a reference in order to secure a meaningful description.
To usefully distinguish particulars from universals, Russell posits the example of "the most long-lived of men," a description which wholly consists of universals. We assume that the description must apply to some man, but we have no way of inferring any judgment about him. Russell remarks, "all knowledge of truths, as we shall show, demands acquaintance with things which are of an essentially different character from sense-data, the things which are sometimes called 'abstract ideas', but which we shall call 'universals'." The description composed only of universals gives no knowledge by acquaintance with which we might anchor an inference about the longest-lived man. A further statement about Bismarck, like "The first Chancellor of the German Empire was an astute diplomatist," is a statement that contains particulars and asserts a judgment that we can only make in virtue of some acquaintance (like something heard or read).
Statements about things known by description function in our language as statements about the "actual thing described;" that is, we intend to refer to that thing. We intend to say something with the direct authority that only Bismarck himself could have when he makes a statement about himself, something with which he has direct acquaintance. Yet, there is a spectrum of removal from acquaintance with the relevant particulars: from Bismarck himself, "there is Bismarck to people who knew him; Bismarck to those who only know of him through history" and at a far end of the spectrum "the longest lived of men." At the latter end, we can only make propositions that are logically deducible from universals, and at the former end, we come as close as possible to direct acquaintance and can make many propositions identifying the actual object. It is now clear how knowledge gained by description is reducible to knowledge by acquaintance. Russell calls this observation his fundamental principle in the study of "propositions containing descriptions": "Every proposition which we can understand must be composed wholly of constituents with which we are acquainted."
Indirect knowledge of some particulars seems necessary if we are to expressively attach meanings to the words we commonly use. When we say something referring to Julius Caesar, we clearly have no direct acquaintance with the man. Rather, we are thinking of such descriptions as "the man who was assassinated on the Ides of March" or "the founder of the Roman Empire." Since we have no way of being directly acquainted with Julius Caesar, our knowledge by description allows us to gain knowledge of "things which we have never experienced." It allows us to overstep the boundaries of our private, immediate experiences and engage a public knowledge and public language.
This knowledge by acquaintance and knowledge by description theory was a famous epistemological problem-solver for Russell. Its innovative character allowed him to shift to his moderate realism, a realism ruled by a more definite categorization of objects. It is a theory of knowledge that considers our practice of language to be meaningful and worthy of detailed analysis. Russell contemplates how we construct a sense of meaning about objects remote from our experience. The realm of acquaintance offers the most secure references for our understanding of the world. Knowledge by description allows us to draw inferences from our realm of acquaintance but leaves us in a more vulnerable position. Since knowledge by description also depends on truths, we are prone to error about our descriptive knowledge if we are somehow mistaken about a proposition that we have taken to be true.
Critics of this theory have held that Russell's hypothesis of knowledge by description is confusing. His comments when defining sense-data, that the physical world is unknowable to us, contradict his theory of knowledge by descriptions. He implies that "knowledge by description" is not really a form of knowledge since we can only know those things with which we are acquainted and we cannot be acquainted with physical objects. Russell's theory amounts to the proposition that our acquaintance with mental objects appears related in a distant way to physical objects and renders us obliquely acquainted with the physical world. Sense-data are our subjective representations of the external world, and they negotiate this indirect contact.
While innovative, Russell's theory of knowledge by description is not an attractive theory of knowledge. It is clearly unappealing because our impressions of the real world, on his view, are commensurate with muddy representations of reality. Though we have direct access to these representations, it seems impossible to have any kind of direct experience of reality. Reality, rather, consists in unconscious, inferential pieces of reasoning.