The character Twain was writing about, Pudd'nhead Wilson, is trying to define courage by giving examples of what it is and what it isn't.
Courage IS resistance to fear
Courage IS mastery of fear
But courage IS NOT absence of fear
He goes on to give the example of the flea, a tiny, fragile, creature that lives on creatures that are much bigger and stronger than it is, that could crush it instantly. He says that if this definition of courage were not true then the lowly flea would be the bravest of animals.
So, short answer, 'This is that' does not need an article.