Who is this person?

[Anita]

Time to reveal that the person Lincoln was writing is Isaac N. Morris.

The reason for the "sincere thanks" is apparent here.

ALS, owned by Edward C. Stone, Boston, Massachusetts. On December 17, Isaac N. Morris, Democratic member of congress from Quincy, Illinois, had offered a resolution in the House of Representatives, reading in part as follows: `` . . . That we properly estimate the immense value of our national Union to our collective and individual happiness . . . cherish a cordial . . . attachment to it . . . will watch its preservation with jealous anxiety . . . that we have seen nothing in the past, nor . . . present, either in the election of Abraham Lincoln to the Presidency . . . or from any other existing cause, to justify its dissolution; that we regard its perpetuity as of more value than the temporary triumph of any party or any man. . . . '' (Illinois State Journal, December 19, 1860). The resolution passed 115 to 44.
Collected Works of Abraham Lincoln. Volume 4.
Lincoln, Abraham, 1809-1865.

https://thelincolnlog.org/Results.aspx?t...1860-12-24