There are collections too big to be a set, e.g. the collection of all sets (in ZFC), and there are collections that cannot be sets for "pure" logical reasons, e.g. the collection of sets that do not contain themselves.

There are functions growing too fast to be computable, e.g. the busy beaver function, and there are functions that cannot be computed for "pure" logical reasons, e.g. the halting function.

In the final end it is of course shown by logical means that being too big (growing too fast) prohibit a collection to be a set (a function to be computable), but those properties are not "purely" logical (they are about sizes and growth rates), opposed to the "pure" logical reasons mentioned above.

> Is there a simple lesson to be learned
> from these observations? Are there 
> other not "purely" logical properties
> of collections (functions) that
> prohibit them to be sets (computable)?