Rationalism provides the counterpart to empiricism (knowledge through experience and sensory data) as a way to justify true beliefs as knowledge. It relies heavily on rationale, logic and deductive reasoning and centers around three key theses; that of intuition and deduction, innate knowledge and innate concepts.
The intuition/deduction thesis is centered around the idea that a proposition can be knowable if it can be reached via deduction from points of innate knowledge. For example, we innately know that the number two is greater than one, and that it is even. From that, we may deduce that there exists an even number greater than one, and that deduction can be considered knowledge. Contemporary rationalists must however, be aware of the potential for some less obvious ‘intuitions’ to be mistaken.
Sherlock Holmes is a famous literary example of a strong advocator of deductive reasoning.
The innate knowledge and innate concepts theses pertain to the first part of the intuition/deduction thesis’ rationale - a priori part of that equation. They posit that some knowledge and concepts - pain being the result of an injury, for example - are available to humanity innately due to our rational nature and experience only brings this knowledge into the forefront of our minds. This thesis finds its roots in Plato’s Meno but it is hotly debated by empiricists and some schools of rationalism today.