Though much is taken, much abides; and though
We are not now that strength which in old days
Moved earth and heaven; that which we are, we are;
One equal temper of heroic hearts,
Made weak by time and fate, but strong in will
To strive, to seek, to find, and not to yield.
The Patronus is a kind of positive force, a projection of the very things that the dementor feeds upon — hope, happiness, the desire to survive — but it cannot feel despair, as real humans can, so the dementors can’t hurt it.