Kernel
Leibniz is the figure who first formulates the question this archive ends on: "why is there something rather than nothing?" (1697). He is also one of two independent inventors of calculus, the inventor of binary arithmetic, the designer of mechanical calculators that anticipated modern computing by two centuries, and the author of a monadology that proposed mind-like atoms as the fundamental substance of reality.
Contribution
Calculus (concurrent with Newton). Binary arithmetic. The principle of sufficient reason. The Monadology (1714). Pre-established harmony. The mathematical infrastructure of early modern philosophy and the foundational vocabulary of every subsequent debate about possibility, necessity, and modality.
Civilization-scale significance
If Newton is the figure of mathematized physics, Leibniz is the figure of mathematized metaphysics. The principle of sufficient reason — that everything has a reason for being the way it is — is the philosophical commitment that 18th-century Enlightenment rationalism runs on.