Russell is now able to develop a theory of significance. Regarded epistemologically, a proposition has two sides, objective and subjective. The objective side is what it indicates factually. The subjective side is what it expresses about the state of mind of its originator; this is called its significance. What it expresses may be belief, denial, or doubt. These distinctions, not needed in logic, solve many puzzles of epistemology. The points concerning significance are independent of truth or falsity of the proposition; truth and falsity come into the relation of the proposition to what it indicates. A proposition does not necessarily consist of words; it is psychological, of the stuff of belief, not language. However, words may always be found to state the belief that, as a proposition, may underlie the many possible ways of saying it, in one or in various languages. Russell provides a sample language to show that the psychological conditions of significance can be translated into precise syntactical rules.
Logical sentence patterns can start from particular propositions recording percepts and extend one’s thought over material that one has not experienced, and in this way one can expand one’s body of statement. If one knows “Socrates is mortal,” one can think “Something is mortal,” or “Everything is mortal,” and so on. Then further inquiry so as to have new percepts may test whether the new statements should be added to belief. Simple statements of immediate percepts may be expressed with constants—particular names—and predicates. However, any statement covering a percept one has not actually had must contain a variable term in place of a constant, for one can neither give nor learn a name (in Russell’s sense) for an object one has not perceived. An epistemological language will need names, whereas a logical language does not deal with particulars and has no use for names. By the use of variables rather than names, it is possible to have propositions transcending one’s experience. This is in fact what happens whenever one receives information from another person.
Thus far Russell has investigated meaning. In effect, he has constructed an epistemological language so that one can know what kinds of sentences are possible as statements of percepts and their relationships. It remains to examine the relationship between meaning and truth, between language and the world.