I asked Reddit yesterday but haven't got a reply. And I can't find a clear definition or explanation online for /moderate/ foundationalism. I have to explain why Russell is considered a moderate foundationalist, as per Problems of Philosophy. I'm having difficulties organizing my thoughts and putting them into a coherent answer.

He would be considered one as his arguments are based on central, but quasi-certain, beliefs? Such as the certainty of sense-data, which he then uses to build a less certain argument for the existence of things in themselves.

He seems to be dancing with the idea that these things can't necessarily be proved or disproved, which I presume would be a part of the reasoning behind the classification of him as a moderate foundationalist?