Aufbau der Welt, for instance, might be interpreted as implying that a proposition about future events did not really refer to the future at all but asserted only the present existence of certain expectations (and, similarly, speaking about the past would really mean speaking about present memories). But it is certain that the author of that book does not hold such a view now, and that it cannot be regarded as a teaching of the new positivism.
On the contrary, we have pointed out from the beginning that our definition of meaning does not imply such absurd consequences, and when someone asked, “But how can you verify a proposition about a future event?”, we replied, “Why, for instance, by waiting for it to happen! ‘Waiting’ is a perfectly legitimate method of verification”.
Thus I think that everybody — including the Consistent Empiricist — agrees that it would be nonsense to say, ‘We can mean nothing but the immediately given’. If in this sentence we replace the word ‘mean’ by the word ‘know’ we arrive at a statement similar to Bertrand Russell’s mentioned above. The temptation to formulate phrases of this sort arises, I believe, from a certain ambiguity of the verb ‘to know’ which is the source of many metaphysical troubles and to which, therefore, I have often had to call attention on other occasions (see e.g. Allgemeine Erkenntnislehre 2nd ed. 1925, § 12). In the first place the word may stand simply for ‘being aware of a datum’, i.e. for the mere presence of a feeling, a color, a sound, etc.; and if the word ‘knowledge’ is taken in this sense the assertion ‘Empirical, knowledge is confined to what we actually observe’ does not say anything at all, but is a mere tautology. (This case, I think, would correspond to what Professor Lewis calls “identity-theories” of the “knowledge-relation” Such theories, resting on a tautology of this kind, would be empty verbiage without significance.)
In the second place the word ‘knowledge’ may be used in one of the significant meanings which it has in science and ordinary life; and in this case Russell’s assertion would obviously (as Professor Lewis remarked) be false. Russell himself, as is well known, distinguishes between ‘knowledge by acquaintance’ and ‘knowledge by description’, but perhaps it should be
