This article is from the The Ghost and Mrs. Muir FAQ, by Caroline Kent caro@earthlink.net with numerous contributions by others.
Children see and hear many things that adults seem to have lost the
ability to experience. Ditto for animals. Jonathan was able to see the
Captain from the very beginning, as was his pet dog, Scruffy.
Jonathan's older sister, Candy, however, was unaware of the Captain's
presence until the second season. How and when she was introduced to
Captain Gregg was not explained. In the case of adults, the Captain
makes his presence known only to those whom he so desires to be seen
by. Claymore Gregg (unfortunately for him) is aware of the Captain's
existence. Carolyn was introduced to the Captain shortly after
arriving in Gull Cottage, while Martha didn't meet the Captain until
much later during the show's second season.
 
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