"To see what is in front of one's nose needs a constant struggle." wrote George Orwell in this delightful essay first published in 1946.
Just after this he wrote:
"One thing that helps toward it is to keep a diary, or, at any rate, to keep some kind of record of one's opinions about important events. Otherwise, when some particularly absurd belief is exploded by events, one may simply forget that one ever held it."
The first quote reminds the importance of thinking with one's feet placed firmly on the ground. It is what keeps one firmly connected to the ground and prevents one from being blown away in the winds of our own dreams.
The second suggests that writing down these thoughts helps to record them. One can then go back to these thoughts later to examine one's thought process before some event. This is important because frequently one tends to forget our previously held opinion and believe that we infact thought differently before.