psychology, is a notion incapable of precision. This brings me to the question whether the truth or falsehood of a belief can be determined either by its causes or by its effects. There is, however, a preliminary difficulty. I said just now that A believes _p_ if he acts in a way which will achieve his ends if _p_ is _true_. I therefore assumed that we know what is meant by “truth”. I assumed, to be definite, that we know what is meant by “truth” as applied to a form of words. The argument was as follows: From observation of a person’s acts, you infer his beliefs, by a process which may be elaborate as the discovery of Kepler’s Laws from the observed motions of the planets. His “beliefs” are not assumed to be “states of mind”, but merely characteristics of series of actions. These beliefs, when ascertained by observation, can be expressed in words; you can say, _e.g._ “This person believes that there is a train from King’s Cross at 10 A.M.” Having once expressed the belief in words of which the meaning is known, you have arrived at the stage where formal theories are applicable. Words of known meaning, put together according to a known syntax, are true or false in virtue of some fact, and their relation to this fact results logically from the meanings of the separate words and the laws of syntax. This is where logic is strong. It will be seen that, according to what we have said, truth is applicable primarily to a form of words, and only derivatively to a belief. A form of words is a social phenomenon, therefore the fundamental form of truth must be social. A form of words is true when it has a certain relation to a certain fact. What relation to what fact? I think the fundamental relation is this: a form of words is true if a person who knows the language is led to that form of words when he finds himself in an environment which contains features that are the meanings of those words, and these features produce reactions in him sufficiently strong for him to use words which mean them. Thus “a train leaves King’s Cross at 10 A.M.” is true if a person can be led to say, “It is now 10 A.M., this is King’s Cross, and I see a train starting”. The environment causes words, and words directly caused by the environment (if they are statements) are “true”. What is called “verification” in science consists in putting oneself in a situation where words previously used for other reasons result directly from the environment. Of course, given this basis, there are innumerable indirect ways of verifying statements, but all, I think, depend upon this direct way.