A filter can be good in a couple of ways: it can filter out ~all of what you don't want, and/or ~none of what you do want. Time is a good filter in the first w

A filter can be good in a couple of ways: it can filter out ~all of what you don't want, and/or ~none of what you do want. Time is a good filter in the first way, which makes it a good filter. Because a filter which doesn't substantially do the first of these things isn't actually filtering: the null filter filters none of what you do want, by failing to reduce the data stream in any way.