Why doesnt Scrooge not believe in the supernatural occourences before him

Scrooge explains his own disbelief as indigestion;

‘You don’t believe in me,’ observed the Ghost.

‘I don’t.’ said Scrooge.

‘What evidence would you have of my reality beyond that of your senses?’

‘I don’t know,’ said Scrooge.

‘Why do you doubt your senses?’

‘Because,’ said Scrooge, ‘a little thing affects them. A slight disorder of the stomach makes them cheats. You may be an undigested bit of beef, a blot of mustard, a crumb of cheese, a fragment of an underdone potato. There’s more of gravy than of grave about you, whatever you are!’