Flickers

Where do thoughts come from?  I know that sounds like a child's question - but I cannot resolve how certain thoughts insinuate themselves.  Yesterday afternoon when I was trying to organise myself I couldn't stop thinking about someone.  I wanted to, but whenever I established a new train of thought, he would seep up through the cracks and change the direction of my thinking.  Perhaps it is when one is wool-gathering - things just crop up.  I know that certain places of ideas are linked to certain things - e.g. if someone said Blenheim, one might think of Marlborough - or Winston Churchill, or Capability Brown - but what about people who suddennly crop up without any particular route... is it telepathy?  Sometimes these thoughts occur and a little later one has an email or a phone call from them.   

And what is that other (related?) phenomenon when unconscious thoughts of someone just ripple through one's mind as one is preparing to sleep?  I associate a lot of this with times when I've been in love, but it is disturbing when it happens apparently spontaneously.  Perhaps my mind's just shedding futile ideas?