Full Idea
The greatest knower of them all may yet not know the whole of everything, or even know what he does know at one single stroke: - he may be liable to forget.
Gist of Idea
If there is a 'greatest knower', it doesn't follow that they know absolutely everything
Source
William James (Pragmatism - eight lectures [1907], Lec 4)
Book Reference
James,William: 'Pragmatism - eight lectures' [Dover 1995], p.56
A Reaction
And that's before you get to the problem of how the greatest knower could possibly know whether or not they knew absolutely everything, or whether there might be some fact which was irremediably hidden from them.