I had a cracking example of this the other night. My 8 year old daughter, at bedtime ran and hid from me. I thought she had probably got into my bed. There was a lump under the covers - small and not obviously a person but possibly where she was. I went to pull back the covers. I knew that this would either reveal my daughter or an empty bed. There was no other possibility. And yet as i reached out to pull back the covers I was very briefly but intensely awash with uncanny anxiety because somewhere in be existed an uncertainty and in that uncertainty existed the possibillity that there could be almost anything under those covers.