In a conversation some things are known, some are partially known and others are hidden. It's a bit like looking at a large picture in the dark using a torch. We can see the picture in the centre of the beam quite clearly. As we move towards the edge of the beam things are not quite so clear but we can still make out that there is something there. And beyond the beam there is darkness but we can see that we have not seen the edge of the picture.

Conversations are a bit more tricky as we don't have a nice clean edge to what is relevant. We have to make a judgement about when something relevant, or not. And, of course, what may not seem relevant to me might be very relevant to you.