A more thorough explanation:

RATIO PERTINENS

A pertinent reason (for a question).

Ratio pertinens is a Latin term that means "pertinent reason." It refers to a reason that is relevant or applicable to a particular question or situation. In other words, it is a reason that directly relates to the matter at hand.

• When asked why he was late for work, John gave a ratio pertinens by explaining that his car had a flat tire on the way.
• During a court trial, the lawyer presented a ratio pertinens to support his argument and prove his client's innocence.

Both examples illustrate how a ratio pertinens is a reason that directly relates to a specific question or situation. In the first example, John's reason for being late is directly related to the question of why he was late for work. In the second example, the lawyer's reason is directly related to the question of whether or not his client is guilty.