I get to know the external world through my sense-perceptions. It is only through them that such knowledge flows into me; they are the very material out of which I construct it. The same applies to everyone else. The worlds thus produced are, if we allow for differences in perspective, etc., very much the same, so that in general we use the singular: world. But because each person's sense-world is strictly private and not directly accessible to anyone else, this agreement is strange; what is especially strange is how it is established. Many people prefer to ignore or gloss over the strangeness of it, explaining the agreement by the existence of a real world of bodies which are the causes of sense-impressions and produce roughly the same impression on everybody.

But this is not to give an explanation at all; it is simply to state the matter in different words. In fact, it means laying a completely useless burden on the understanding. The broad measure of agreement between two observed worlds, let us say B and B′, is to be explained by some sort of correspondence with the real world, R, that is of B with R and of B′ with R. Anyone who thinks like this is forgetting that R has not been observed. No one perceives two worlds, one observed and one ‘real’, no one is in a position to establish any sort of similarity in their structure.