It is not intended here to emphasise the difference between facts verifiable by experiment, and historical facts; yet it is desirable with reference to all kinds of facts stated on authority, to understand the importance of a criterion of truth. We do accept and we have to accept, every one of us, without any exception, the most discriminate scientist even and most of all the philosopher, innumerable statements of facts as they have been observed by others. We all have to rely on the authority of others. The time of the longest human life would be too short to repeat all the experiments made by others, with a view to verifying them in detail. On the other hand, it is obvious that no statement of facts should be accepted on pure authority. We must have a means, a sieve as it were, by which the wheat can easily be winnowed from the chaff; a sieve that will enable us to discard at once those statements that are positively erroneous. In this way our attention can be confined to statements of things that are possible, those that need not, but may be true. "Possible" in German is very appropriately called möglich, i. e. mayable.
The criterion of that which 'may be' true is the first step towards ascertaining truth; and although it does not exhaust the methods of arriving at truth it is of greatest consequence, for if properly understood and applied, it would save from the start many useless efforts in the investigation of truth.
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The question arises then, What is the criterion of the possible? We reject statements, sometimes, as prima facie untrue. Have we a right to do so? And if we have, by what standard do we determine this?
Let us first take into consideration how people really behave when a statement of new facts is made. Take, for instance, the following case. Two strangers meet; A. and B. Mr. A. relates to Mr. B. some incident of his life. He is apparently a very trustworthy person and during the conversation remains perfectly serious. He tells a ghost story in detail, how a departed friend of his appeared to him in distinctly visible form; he says that the spirit spoke to him and told him many strange things, and that he pointed out to him an imminent danger.
We suppose that on the one hand A. makes his statement in good faith and that on the other hand B. is a spiritualist. Will B. consider A.'s story as possible? B., being a spiritualist, most probably will consider A.'s story as possible, and, if he is convinced of A.'s honesty, he will believe the story the same as if he had experienced it himself; no less than a scientist will rely on the statement of an experiment made by one of his colleagues whose scientific veracity he has no reason to doubt.
Suppose A. tells the same story to C. Mr. C. is an infidel and a materialist. As characteristic features of his personality we might mention that he considers religion as pure superstition originated by the fraud of cunning priests. This man will, we may fairly suppose, laugh at A.'s story, because it appears to him an out and out lie. Mr. A. as well as Mr. B., he who tells and he who believes the story, C. will declare, are either insane or they are both impostors.
The difference of opinion in B. and C. indicates that the criterion of truth is different with different persons and that it depends upon their conception of the world. Men who have the same world-conception will also have the same criterion of truth.
The problem consequently is, whether this criterion of truth (i. e. the criterion of what is possible) is necessarily wholly subjective, or whether we can arrive at an objective criterion. It is apparent that this question is intimately connected with another problem, namely, Is every world-conception necessarily subjective, or, Is it possible to arrive at an objective world-conception? It appears to me that we can; and the ideal of philosophy to-day is just such an objective representation of facts.
The difficulty that presents itself lies mainly in the confusion between facts and our interpretation of facts. If A. declares that he saw a ghost, he does not relate a fact, but his interpretation of a fact. Let us suppose that he tells his story again to a third person D., who is a psychologist. D. most likely will not think him a liar. D. will accept the statement bona fide as a mere interpretation of a fact and will inquire after the causes that produced the hallucination. He may be able, possibly, to lay bare the facts disfigured by the wrong interpretation of A. And having clearly stated the objective state of things he may with the assistance of his experience explain the origin of the whole process, partly from the mental condition and the physiological constitution of A., partly from individual circumstances that gave rise to the hallucination. He will not doubt that something extraordinary has happened to Mr. A. The latter's mind has been, and perhaps still is in an abnormal state. And as to B.'s believing the ghost story, Mr. D. will not think that he is insane; though we may presume that he will regard B.'s views of the world as resting upon unfirm grounds; and he will not believe him to be a man of critical ability.