I agree that the phrase is less than felicitous; however, 'fairly certain' is an extremely common collocation (53,000 hits on Google Books), so logic – once again – is trumped by usage.
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Anonymousthe question was if we could write or say "probably certain". You answered that we could say "fairly certain".No, I did not; I observed what has been used. I did imply that both 'probably' and 'fairly' are gradings, however.
Anonymous "probably certain" as a phrase as only one hit on Google: this post on this forum