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Let me propose a different definition.
Let us say that a man is certain of a given proposition if and only if two conditions are fulfilled, first, that he has not the slightest genuine doubt that it is true, and secondly that it really is true.
In that very useful sense, a man will not be obliged to carry his criticisms of reasoning very far to be quite certain of its soundness, that is, that the conclusion can not be false if the premisses are true.
At any rate, after a man has once carried his criticism of reasoning so far that upon mature and careful scrutiny not the slightest shade of doubt of it can be awakened in his mind, he certainly can go no further for the time being.
For he will not then have the slighest indication of what point these may be in it that can even appear doubtful.
In this way