Another extremely natural answer arises, of course, with Spinoza, many of whose arguments follow a deductive axiomatic style, with formally stated axioms, definitions, theorems and corollaries.
http://upload.wikimedia.org/wikipedia/commons/e/ea/Spinoza.jpg
Here is what Lucian Wischik has to say about Spinoza's ethics:
One of the most remarkable features of the Ethics is its axiomatic form. Spinoza sets out at the start a small number of definitions and axioms that are assuredly true, and proceeds to deduce from these the rest of his philosophy. In this respect, the work is an attempt to use a theory of philosophy that is modelled upon Euclid’s Elements.
Here is Charles Jarrett's article for the Canadian Journal of Philosophy on Spinoza's ontological argument.
Here are A. Pruss's lecture notes on Spinoza, in which he asserts:
Spinoza’s approach is geometrical, that is modeled on the reasoning in geometry. Euclid defined various terms, provided axioms, and everything else was to be proved from the axioms and definitions.