Is Methodological Naturalism an Empirical Discovery, or an A Priori Constraint?

Much of the debate surrounding Intelligent Design vs. Neo-Darwinism comes down to the definition of science itself. Critics often argue that ID is not science because it appeals to an intelligent agent rather than a purely physical mechanism.

But science already routinely infers intelligent agency as a valid explanation in fields like:

• Forensics (distinguishing accidental death from homicide)
• Archaeology (distinguishing naturally weathered rocks from arrowheads)
• SETI (looking for ordered radio signals from space)

So if inferring design is considered a legitimate scientific method in archaeology and forensics when observing highly improbable, functional patterns, why should biological systems be strictly disqualified a priori from the same diagnostic criteria?