Imagine two business executives who each suddenly realise the truth of

(1) The meeting begins in five minutes.

Executive A knows that it is now five minutes to ten, but thought the meeting was at eleven, and has just recognised the truth. Executive B knows that the meeting is at ten, but thought the time was only half past nine. B's ignorance may be expressed by saying that B has just realised the truth of

(2) It is five minutes to ten.

(2) is an interesting sentence in terms of its empirical content. Contrast it with (1). For things to be as executive A thinks they are the world would have to be different. For things to be as B thinks they are the world is the same but the time is different. In the case of (2) there is no comparable distinction. If it is five minutes to ten there is no world in which it is not then five to ten. For such a world would be one in which at five to ten it is not five to ten. What this means is that a correct account of the objects of knowledge has to treat them as tensed propositions. (1) is true at 〈w,t〉 iff

(3) The meeting begins

is true at 〈w,t′〉 where t precedes t′ by five minutes. (2) is true iff t is five minutes to ten.