In a topos, the question is not whether a sentence is true or false, but where it is true, because toposes  -- at least the geometric ones you're talking about  -- are generalized spaces. Usually, there are some limit points where this question cannot be decided for all sentences. The problem is then that other points can be found within any distance (more accurately: any open neighborhood) both where a sentence is true and where the same sentence is false.

In a topos, the question is not whether a sentence is true or false, but where it is true, because toposes--at least the geometric ones you're talking about--are generalized spaces. There can be limit points where this question cannot be decided for all sentences. The problem is then that there are other points within any distance (more accurately: any open neighborhood) both where a sentence is true and where the same sentence is false.