An individual's father's cousin is the individual's second cousin, assuming the cousin is the child of one of the father's siblings. Familial relationships are determined by descent beginning with the first common ancestor or relative.
Cousins are people who are born to siblings. Those siblings are separate sets of individuals, so working up the family tree, the next set of relatives are the grandparents (the parents of a set of cousins' parents). Since the child of a sibling and that sibling's cousin must travel backward two spots to find a common ancestor in the grandparents for the cousin and the great grandparents for the child of the sibling, they are second cousins.