Disputes about matters of fact are disputes about the conditions for truth, in this way every such matter affects every other. It is not certain that the conditions for truth can be recognised, they can only be enacted hypothetically to see if they fit, the recognition is deferred and anyway left to you. If there was a way of knowing one true thing then it could be used to slit open every semblance, but you cannot be certain that you yourself contain a condition for truth, much less the thing itself.