Let:
- M: a miracle happened
- ~M: no miracle happened
- R: reports of a miracle
The posterior probability of a miracle given reports of it is expressed as:
P(M|R) = P(R|M) * P(M) / P(R)
Where:
P(R) = P(R|M) * P(M) + P(R|~M) * P(~M)
Substituting, we can simplify to:
P(M|R) = 1 / (1 + (P(R|~M) * P(~M)) / (P(R|M) * P(M)))
Key Considerations
Prior Probability of a Miracle (P(M))
- P(M) is arguably very low since most people seldom witness or report miracles, especially of extraordinary types, like a resurrection, which is extremely rare even among miracle claims.
- Consequently, P(~M) is very high, as it represents the complement of P(M).
Likelihood of Reports Given a Miracle (P(R|M))
- P(R|M) can reasonably be high if the reports align with realistic, plausible reactions expected from individuals witnessing a genuine miracle.
Likelihood of Reports Given No Miracle (P(R|~M))
- P(R|~M) depends heavily on the context of R:
- If ~M holds, R could arise from factors like deception, hallucination, or extraordinary but non-miraculous events (e.g., advanced technology or aliens).
- For mundane scenarios within ~M, consistent and sincere reports of R would be unexpected unless due to improbable coincidences like group hallucination or coordinated deception.
Thus, while P(R|~M) is plausible, its value depends on how we frame the alternatives under ~M.
Effect of Independent Witnesses
On P(R|M):
- Adding more independent, consistent witnesses generally increases P(R|M), especially if their reports align with what we would expect to observe if a miracle occurred. This value can approach 1 under these conditions.
On P(R|~M):
- The effect depends on the specific alternatives within ~M:
- If ~M includes scenarios like aliens or hidden advanced technology, adding witnesses could make P(R|aliens or technology) more plausible.
- However, ~M also includes mundane scenarios, where consistent reports of R would remain highly unlikely.
- Since ~M spans a vast set of possibilities, the fraction of scenarios that make R likely is small compared to those where R is improbable.
Net effect: It’s unclear whether P(R|~M) decreases significantly with more witnesses, as it depends on the weighting of scenarios within ~M.
Final Thoughts
Estimating key terms in these calculations is complex and heavily reliant on the assumptions made about ~M. If we are open to considering miracles, we might also entertain scenarios involving aliens, advanced technology, or other extraordinary explanations. These alternatives can meaningfully affect our probability estimates in non-trivial ways.