Oil on Canvas — 16″x 16″
Shakespeare was a logical man, therefore, he went about coining sensible phrases in a rational fashion. Thus, Hamlet does not say “in my heart of hearts,” but “in my heart of heart,” since we only have one heart.
The phrase is in fact a synonym for 'In my heart’s core.' If you believe or know something in your heart of heart, that is what you really believe or think.