Clash of Kings by George R. R. Martin
Soon they were high enough so that looking down was best not considered. There was nothing below but yawning blackness, nothing above but moon and stars.
The author meant that you shouldn't have looked down because of this white-knuckle height. I understand that this can be paraphrased: it was best not to consider it
But what is the grammar behind this structure? I've found that there is a structure "(had) best do something". You can use it in the past like in this example: You had best have closed that door. But I couldn't find the phrase used in the book (was best not considered).
When thinking on it some time, I came up with some more phrases which are similar to the one in the book:
1.It was best not to be considered 2.It was best not to consider it
The 2nd variant sounds natural to me. The first seems wrong because to be considered means some obligation. (You must not look down!)
But what about the variant in the book? Can anyone explain it to me in terms of grammar?
Thanks in advance