The other day I posted a question about the Rev. Dr. Thomas Harrison's descendants and while researching the question on-line came across an abstract of his will and reference to a certified copy at the Virginia Historical Society. The original will was filed in 1682 and destroyed in the Four Courts fire in June 1922. I've always thought that somebody, sometime, would have made a copy during 240 years.
I found an article by Francis Burton Harrison in The Virginia Magazine of History and Biography, Vol. 53, No. 4 (Oct., 1945), pp. 302-311 that includes an abstract of the will and a reference to its location at the Virginia Historical Society and have ordered a copy.
No sons are named in the will. While this doesn't prove Thomas was not Isaiah's father, it makes it so very unlikely that research time would be better spent exploring other possibilities.