What is the “rule of reason” in determining conception in patent law?

This page is an FAQ based on guidance from the Manual of Patent Examining Procedure. It is provided as guidance, with links to the ground truth sources. This is information only: it is not legal advice.

What is the “rule of reason” in determining conception in patent law?

The “rule of reason” is a principle applied by courts when evaluating evidence of conception in patent cases. According to MPEP 2138.04, “An inventor’s testimony, standing alone, is insufficient to prove conception unless it is corroborated, a rule that the courts have recognized as the ‘rule of reason’.” This means that:

• An inventor’s uncorroborated testimony alone is not enough to establish conception.
• Courts require additional evidence to support the inventor’s claims of conception.
• The rule ensures that the determination of conception is based on a reasonable evaluation of all available evidence.

The MPEP further states: “The rule of reason, however, does not preclude the possibility of proving conception by an inventor’s testimony which is sufficiently corroborated by independent evidence.” This indicates that while inventor testimony is important, it must be supported by other credible evidence to establish conception in patent law.