In the discussion thread to “Is atheism a default position?” Emilie_dC asked the following question:
“If someone shows me two glasses of milk, and tells me that they are identical except that glass A contains an additional substance that is undetectable, what should I believe?
“Should I be agnostic on the matter of whether glass A contains an extra mysterious substance, or should I believe that glass A does not contain an extra mysterious substance?”
A thought experiment like this is far too thin in detail to answer. And consequently, in present form it can’t tell us anything about the rationality of belief based on testimony. The only way to answer the question is by thickening the description.
Here’s the basic question before us. When two objects or states of affairs appear indistinguishable, can one person come to believe they are different based wholly on the testimony of another person? The answer, surely, is yes.
Let’s consider another scenario with another alleged undetectable substance. Let’s say that the trusted neighborhood handy man is invited by his neighbors (who speak very little English) over to their house to look at their furnace. While there, he discovers that the level of carbon monoxide in the house is dangerously high. Immediately he turns to the home owners: “Leave!” he says, gesturing to the door. “Danger!” They stare back at him in surprise. “Bad gas!” he says, waving his arms around the room.
The couple understand enough English to know that the man is warning them that there is a bad gas in the room. But they don’t smell, see or taste anything strange. The inside of the house looks the same this day as every other day. Yet, because they trust this man’s testimony, they come to believe the house interior presently contains a dangerous undetectable substance that it did not contain previously. And so they speedily evacuate the premises.
Obviously the same point applies to the milk. If it is possible to believe a building contains an undetectable substance based on the testimony of another, it is likewise possible to believe a glass of milk contains an undetectable substance based on the testimony of another.
An objector might interject: “But the gas is detectable! After all, the handyman detected it!” Of course this is true, but it is also irrelevant. The point is that the gas is undetectable to the couple and moreover that the couple lacks the ability (due to language constraints) to detect the gas themselves. Consequently, the couple’s belief that the gas is present is contingent wholly upon the testimony of the handyman.
So what is the relevance of the question and the answer? I’m not entirely sure what Emilie_dC was aiming to accomplish by posing the initial question. But I certainly can offer some reflections. Just as it is possible to have justified belief in an otherwise undetectable substance based on the testimony of another, so it is possible to have justified belief in an otherwise undetectable divine person based on the testimony of another.