A more thorough explanation:

Definition: A priori signals an assertion that is based on prior knowledge or intuition. In Latin, the term literally means ‘from [the] former’. An a priori determination is formed prior to investigation.

For example:

• In McCulloch v. Maryland, the court questioned “how will [it] ascertain, a priori, that a given amount of tax will crush the bank?”
• In Burch v. Louisiana, the court held that it “[did] not pretend the ability to discern a priori a bright line below which the number of jurors participating in the trial or in the verdict would not permit the jury to function in the manner required by [their] prior cases.”

These examples illustrate how a priori is used to describe knowledge or assumptions made before any investigation or evidence is gathered. In the first example, the court is questioning how they can know beforehand that a certain tax amount will harm the bank. In the second example, the court is admitting that they cannot predict beforehand how many jurors are needed for a fair trial.