He was always so busy, working away from home for weeks at a time. Coming in, late, tired, irritable. We understood that what he did was very important, and that in his time at home he needed to rest and recuperate, not to be bothered with the small concerns of children.
Mum was there solid, reliable, tangible, but Dad... Dad was heard about, but rarely seen -- a voice on the phone, a door closing at midnight, a car starting up outside early in the morning before we were fully awake.
You had to believe in him, because Mum said so, but 'Dad' wasn't real to kids of five and six. He was a threat of retribution -- "I'll tell your father what you did", or an unseen benefactor -- "You couldn't have that bike if Dad didn't work so hard", he was a phantom.
Of course, as we got older, his presence got more solid. We were actually up when he got home, he would talk to us now we had conversation, rather than prattle, he became three-dimensional.
But for the first seven or eight years of my life, all I had was a ghost dad.