I’ve long suspected that the filter is evolution itself. Evolution is a goal-less process, nothing more than addition, really. If you have certain things which propagate faster, you end up with more of those things. Is there anything to say that intelligence is the best strategy for that, overall? It certainly seems to be so right now, but that may just be a matter of human perspective. Maybe the vast majority of planets with life end up chugging along happily with nothing but single-celled microbes for billions of years until their sun goes nova.