time limited thoughts
The scenario I am thinking of is in the use of PB for website publishing where I publish news items to my website and want to have an easy mechanism to remove them after a certain period has elapsed. However, I'm sure there are other applications for this type of feature such as helping to keep your brain 'tidy' by automatically removing information that had expired. I guess there would need to be some form of confirmation dialogue to allow the option of deleting or making the thoughts permanent.

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Thomasi commented
I think the area of application is very wide to pop up a thought after a while (if desired or marked for) to check, if it can be deleted or "forgotten". This is also handsome for any kind of document like versions of a help file, old accouning information for your private account or whatever.
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Cornan commented
How about a new feature to "make thought private after [date]" ?
This would automatically eliminate such thoughts from a publishing cycle without actually deleting them.
I would guess that the same would happen if thoughts were "forgotten" after a certain amount of time (I don't use forgotten thoughts much, but I'm sure other people are).