vision. If the contents "blue" and "black" together with their intuitive spatial qualities were not there, there would of course be no observation; so content does play an absolutely essential role. But now the observer formulates the observed fact by enunciating (or writing down) the proposition "the blue line coincides with the black mark". He may think of the words "blue", "black", "coincides" etc. as standing for the contents in his visual field, but after all that has been said in the first lecture we know that his words and the proposition express nothing of it. The statement expresses the structure of the observed fact in the way which we described formerly in regard to some other example, but it does not convey the content blue or any other. Another scientist who hears or reads the statement must immediately fill the communicated structure with some content of his own, or rather, the communication of the structure is even effected only by arousing in his mind some content which has that structure. Contents will rise in his imagination which he will call "a blue line" etc.; but, as we convinced ourselves long ago, we cannot assert that his content is at all similar to that of the first observer — such an assertion would not be false, it would be meaningless. (If the second scientist happens to be blind and deaf, he will nevertheless be able to understand the statement of the first observer, provided he has received a proper training [Helen Keller], for he will be able to imagine some tactual content of the required structure. If the statement of our observer should induce another scientist to repeat the experiment for himself, use the same apparatus, look through the same telescope under the same circumstances; if he then would confirm the statement by saying: "Yes, the blue line coincides with the black mark" — even then it would be nonsense to say that he had the same content as the first observer, although we should most certainly maintain that they are both in possession of the same structure.)
I hope the whole matter has now become completely clear: the empty frame of a hypothetical-deductive system does have to be filled with content in order to become a science containing real knowledge, and this is done by observation (experience). But every observer fills in his own content. We cannot say that all the observers have the same content, and we cannot say that they have not — not because we are ignorant, but because there would be no sense in either assertion.
