Quoting Carl Shulman from about a year ago:

If we condition on having all other variables optimized, I'd expect a team to adopt very high standards of proof, and recognize limits to its own capabilities, biases, etc. One of the primary purposes of organizing a small FAI team is to create a team that can actually stop and abandon a line of research/design (Eliezer calls this "halt, melt, and catch fire") that cannot be shown to be safe (given limited human ability, incentives and bias). If that works (and it's a separate target in team construction rather than a guarantee, but you specified optimized non-talent variables) then I would expect a big shift of probability from "UFAI" to "null."

I'm not sure if he had both math and philosophy in mind when he wrote that or just math, but in any case surely the same principle applies to the philosophy. If you don't reach a high confidence that the philosophy behind some FAI design is correct, then you shouldn't move forward with that design, and if there is only one philosopher on the team, you just can't reach high confidence in the philosophy.