Suppose Red and Green exchange a series of messages thus:
Date
t1
Home (0, 0, 0)
Dear Green,
I hope this finds you as it leaves me.
Yours ever. Red
To which Green replies:
Date
t2
Home (0, 0, 0)
Dear Red,
Thank you for your letter,
dated `t1',
which has just arrived.
Yours ever,
Green
Red then answers Green's Ietter:
Date
t3
Home (0, 0, 0)
Dear Green,
Thank you for your letter,
dated `t2',
which has just arrived.
Yours ever,
Red
To which Green again replies:
Date
t4
Home (0, 0, 0)
Dear Red,
Thank you for your letter, dated `t3', which has just arrived.
Yours ever,
Green
From this exchange of messages, Red knows when he sent his first message, when he received one back, and also knows the date on which Green says he, Green, received the message according to his, Green's, reckoning,
Click here to return to main argument