Full Idea
I claim to give metaphysics geometric demonstrations, assuming only the principle of contradiction (or else all reasoning becomes futile), and that nothing exists without a reason, or that every truth has an a priori proof, from the concept of terms.
Gist of Idea
Metaphysics is geometrical, resting on non-contradiction and sufficient reason
Source
Gottfried Leibniz (Letters to Antoine Arnauld [1686], 1686.07.4/14 XI)
Book Reference
Leibniz,Gottfried: 'The Leibniz-Arnauld Correspondence', ed/tr. Mason,HT/Parkinson,GHR [Manchester UP 1967], p.71
A Reaction
For the last bit, see Idea 12910. This idea is the kind of huge optimism about metaphysic which got it a bad name after Kant, and in modern times. I'm optimistic about metaphysics, but certainly not about 'geometrical demonstrations' of it.
Related Idea
Idea 12910 The predicate is in the subject of a true proposition [Leibniz]