But in the case of affirmation and negation, whether the subject exists or not, one is always false and the other true
. For manifestly, if Socrates exists, one of the two propositions 'Socrates is ill', 'Socrates is not ill', is true
, and the other false.
MENO: I think that what you are saying, Socrates, is very true.
SOCRATES: But if this is true, then the good are not by nature good?
MENO: True; but do you think that there are no teachers of virtue?
Can you say that they are teachers in any true sense whose ideas are in such confusion?
SOCRATES: And in supposing that they will be useful only if they are true guides to us of action--there we were also right?
SOCRATES: And while he has true opinion about that which the other knows, he will be just as good a guide if he thinks the truth, as he who knows the truth?
If, for example, two propositions p and q were self-evident, and it were also self-evident that p and q could not both be true, that would condemn self-evidence as a guarantee of truth.
The next thing to observe (although I do not personally doubt that most of our judgments of perception are true) is that it is very difficult to define any class of such judgments which can be known, by its intrinsic quality, to be always exempt from error.
Nevertheless, it is perhaps true that judgments having a high degree of subjective certainty are more apt to be true than other judgments.
According to this view, any set of propositions other than the whole of truth can be condemned on purely logical grounds, as internally inconsistent; a single proposition, if it is what we should ordinarily call false, contradicts itself irremediably, while if it is what we should ordinarily call true, it has implications which compel us to admit other propositions, which in turn lead to others, and so on, until we find ourselves committed to the whole of truth.
The question of verifiability is in essence this: can we discover any set of beliefs which are never mistaken or any test which, when applicable, will always enable us to discriminate between true and false beliefs?