Leibniz's essay "Meditations on Knowledge, Truth, and Ideas" is a definitive statement of rationalism in epistemology. It helpfully codifies an approach to truth-seeking centered not on the question "for what do I have evidence?" but rather on "what do I understand?"
Rationalism is often thought of as a long-retired historical phenomenon - born with Descartes and withering under the scorching sun of empiricist and Kantian critique. It is certainly true that the classical rationalists can strike present-day readers as ambassadors from a far-off, alien intellectual culture. To a large extent, we have grown impatient with philosophical paths that treat understanding something, or thinking about it in a certain way, as sufficient for knowing (facts) about reality. Thinkers like Leibniz look constantly in danger of substituting a subjective world for the objective one.
What happens when we refrain from drawing the distinction between subjective and objective validity, instead deferring to the criterion of intelligibility to oneself? What about when we deny the duality between concepts and sensations, and instead regard them respectively as more and less lucid perceptions?
A link to the essay: