“Although people cannot define existence and do use the term with some looseness, yet it is possible to give an extensive definition by pointing to the sorts of things that everyone believes to exist. It is still easier to point to the sort of entities that people agree in believing not to exist, and happily, this is really what concerns us most. People mean by saying that causation only applies to things that exist that it applies, if at all, to what can change; and they believe that, if anything can change, it is things like chairs and tables and minds, and not those like the propositions of Euclid and the multiplication table. What would be agreed then is that, if causal laws apply to anything it is to what can change in so far as it changes.”

Source: Small Gods

Blue Like Jazz (2003, Nelson Books)

Edie : Girl On Fire (2006)
Context: But I really, since I exist, at all, I believe that it's possible for people... I've lived through impossible situations. So I believe in it. I just believe, and that's the magic... That's the whole thing, you talk about magic that there's to believe in, and it is there. But most people don't really believe in it. And I refuse, like, since I'm still alive and done the things I've done and seen things and understood things as far as I have, and I am alive, I mean physically intact. When I shouldn't be, according to medical reports and so forth. I mean I should be, not here. That's all there is to it. So the magic's working and it's a rare situation.

Source: Letters to a Young Poet

Article "The Worst Man in the World" in The Sunday Dispatch (2 July 1933); quoted in The Magical Revival (1972) by Kenneth Grant.
Context: Black magic is not a myth. It is a totally unscientific and emotional form of magic, but it does get results — of an extremely temporary nature. The recoil upon those who practice it is terrific.
It is like looking for an escape of gas with a lighted candle. As far as the search goes, there is little fear of failure!
To practice black magic you have to violate every principle of science, decency, and intelligence. You must be obsessed with an insane idea of the importance of the petty object of your wretched and selfish desires.
I have been accused of being a "black magician." No more foolish statement was ever made about me. I despise the thing to such an extent that I can hardly believe in the existence of people so debased and idiotic as to practice it.

Source: The Expanse, Tiamat's Wrath (2019), Chapter 34 (p. 357)

Source: Jonathan Strange & Mr Norrell

Source: The Veneration of Life: Through the Disease to the Soul (1999), p. 54

13 February 1945.
Disputed, The Testament of Adolf Hitler (1945)