People seem able to get to thirdness from someone telling them. Otherwise how do we have collective thirdness about places like the North Pole. We may each have a slight variation on it - we may see a flag, a sign post, a pile of rocks. But we collectively agree the North Pole exists - just like we accept Mars exists. We will never go there, see it but someone will describe it to us. People will even pay taxes to send rockets to it.

Black holes are a strange form of thirdness. People use it to ask people whether they believe in black holes. As if there is a higher form of thirdness.