“The answer to the ancient question "Why is there something rather than nothing?" would then be that ‘nothing’ is unstable.”

Warum ist überhaupt Seiendes und nicht vielmehr Nichts? Das ist die Frage.
What is Metaphysics? (1929), p. 110
Cf. Gottfried Leibniz, De rerum originatione radicali (1697)ː "cur aliquid potius extiterit quam nihil."
Source: Introduction to Metaphysics

De rerum originatione radicali (1697); reprinted in God. Guil. Leibnitii Opera philosophica quae exstant latina, gallica, germanica omniaː 1 http://books.google.gr/books?id=Huv3Q0IimL0C&vq= (1840), p. 148
Cf. Martin Heidegger, What is Metaphysics? (1929)ː "Warum ist überhaupt Seiendes und nicht vielmehr Nichts? Das ist die Frage."

(quote at 42:11 of 1:56:02)

Source: The Sacred Depths of Nature (1998), p. 167
Context: We are, each one of us, ordained to live out our lives in the context of ultimate questions, such as:
Why is there anything at all, rather than nothing?
Where did the laws of physics come from?
Why does the universe seem so strange?
My response to such questions has been to articulate a covenant with Mystery. Others, of course, prefer to respond with answers, answers that often include a concept of god. These answers are by definition beliefs since they can neither be proven nor refuted. They may be gleaned from existing faith traditions or from personal search. God may be apprehended as a remote Author without present-day agency, or as an interested Presence with whom one can form a relationship, or as pantheistic — Inherent in All Things.
The opportunity to develop personal beliefs in response to questions of ultimacy, including the active decision to hold no Beliefs at all, is central to the human experience. The important part, I believe, is that the questions be openly encountered. To take the universe on — to ask Why Are Things As They Are? — is to generate the foundation for everything else.

The Grand Design (2010)

Source: Green Mars (1993), Chapter 8, “Social Engineering” (p. 401)