1. In advance, that is, without prior investigation or experience.
2. Presumptive, without examination.
3. Literally, from the former; from what comes before, from cause to effect, deductively [applied to reasoning]; from the general to the particular; self-evident, known independently of experience.
4. Etymology: from a-, ab- + prior, "former".

In a priori reasoning, one works from a known premise (or premises) to a reasonably assumed effect. By extension the phrase is also used to mean, "to the best of one's knowledge". This type of reasoning is the foundation of Western logic, especially that which is embodied in syllogism. Someone has stated that "the great masters of a priori reasoning were Aristotle and Sherlock Holmes."