I think that quotation by itself refers just to the Great Silence, and there seem to be two essential possibilities concerning this silence, both of which the a

I think that quotation by itself refers just to the Great Silence, and there seem to be two essential possibilities concerning this silence, both of which the author addresses: Either there is a sterilizing filter which lies ahead of planet bound species such as ours, preventing explosive colonization; Or, there is a psychological filter which "prevents" higher civilizations from making their existence visible - the author addresses this when discussing the Common Zoo hypothesis. Within a single galaxy you might entertain the possibility that we are just the first occurrence of intelligence to emerge. However: "Consider, for example, that the energy of a single star might power an intermittent very narrow-band signal detectable to pre-explosive life like ours across the entire universe [Gott 82]." So we would have to be a cosmically nascent intelligence. That makes the absence of both a biological and psychological filter exceedingly improbable.