A Priori

The Latin term a priori refers to knowledge that comes from theoretical reasoning, rather than from actual observation or personal experience. In the term’s most basic use, a person could assume that, if Bobby went to Kindergarten at least six days, he went to Kindergarten more than five days. In its practical use, however, a priori describes knowledge that exists without reference to any actual proof or reality. To explore this concept, consider the following a priori definition.

Definition of A Priori




  1. Existing in the mind, independent of observance or experience.
  2. Relating to what may be known through an understanding of how certain things work, rather than by observation or experience.


1645-1655       Latin (literally, from what is before)

History of A Priori in Western Law

A priori has existed as a philosophical topic since German philosopher Immanuel Kant explored the fundamental concepts of the structure and experience of the human mind. Kant discussed a priori knowledge as knowledge held by reasoning, independent of actual observation or experience. By contrast, a posteriori (literally, from what is after) refers to knowledge that is gained or held by experience, observation, or other actual proof.

A Priori in the Law

As the term a priori applies to the law, it refers to deductive reasoning, or an idea that is taken as a given. An a priori assumption may be brought out in a legal complaint, motion, or even at trial, as one party’s line of reasoning stems from something that has happened in the past.

For example:

Naomi has filed a civil lawsuit against her employer, Ampco, claiming that she was wrongfully fired from her job. Naomi, who had worked as an administrative assistant at Ampco for about five years, had begun frequently arriving at work late, her excuses becoming more and more far-fetched.

One day, Naomi was involved in an accident on her way to work and, because she was transported to the hospital by an ambulance, nobody called her employer. Naomi’s supervisor assumed, a priori, that Naomi was simply very late again, and fired her.

In Naomi’s example, her employer made an assumption based on her own prior behavior, even though he had no personal knowledge of the situation that actually made her miss work that day. Other a priori assumptions are based on basic truths held by man in general.

For example:

Earthcare Organization has filed a civil lawsuit against Farmer Bob, claiming that his use of pesticides on his property is responsible for killing fish in a nearby stream. The claim is based on a neighboring farmer’s finding of several dead fish in the stream. Bob denies the use of fish-killing pesticides, or any pesticides near the stream, and points out that the dead fish likely floated downstream and became entangled in the beaver dam that partially obstructs the waterway at his property.

Although Farmer Bob has no personal knowledge of where the fish originated, having not witnessed any dead or dying fish, his a priori statement relies on a long-known fact that water runs downhill, or that objects in a stream flow with that water to its destination. In the absence of proof that the fish were poisoned where the stream passes Farmer Bob’s property, the court rules in Farmer Bob’s favor.

Related Legal Terms and Issues

  • Civil Lawsuit – A lawsuit brought about in court when one person claims to have suffered a loss due to the actions of another person.
  • Deductive Reasoning – The logical process of reaching a conclusion based on a coming together of multiple facts or premises that are generally assumed to be true.