Leibniz

" * Identity/contradiction. If a proposition is true, then its negation is false and vice versa.

* Identity of indiscernibles. Two things are identical if and only if they share the same properties. Frequently invoked in modern logic and philosophy.

* Sufficient reason. "There must be a sufficient reason [often known only to God] for anything to exist, for any event to occur, for any truth to obtain." (LL 717).

* Pre-established harmony. See Jolley (1995: 129–31), Woolhouse and Francks (1998), and Mercer (2001). "[T]he appropriate nature of each substance brings it about that what happens to one corresponds to what happens to all the others, without, however, their acting upon one another directly." (Discourse on Metaphysics, XIV) A dropped glass shatters because it "knows" it has hit the ground, and not because the impact with the ground "compels" the glass to split.

* Continuity. Natura non saltum facit. A mathematical analog to this principle would go as follows. If a function describes a transformation of something to which continuity applies, then its domain and range are both dense sets.

* Optimism. "God assuredly always chooses the best." (LL 311).

* Plenitude. "Leibniz believed that the best of all possible worlds would actualize every genuine possibility, and argued in Théodicée that this best of all possible worlds will contain all possibilities, with our finite experience of eternity giving no reason to dispute nature's perfection." (From Plenitude.)"

This is taken from:

Gottfried Leibniz.

This German Philosopher is taken more serioulsy now than in his time. Newton´s Absolute Conception of Time, was preferred for many years over the relationist view of Leibniz, which seems to be better for The Theory of Quantum Gravity".

One simple observation on the modern importance of this thinker, is the size of the Liebniz article in Wikipedia linked above.

Two more things, Leibniz writes:

"1. All our ideas are compounded from a very small number of simple ideas, which form the alphabet of human thought.
2. Complex ideas proceed from these simple ideas by a uniform and symmetrical combination, analogous to arithmetical multiplication."

This rings true.