it has the form "we do not know X for certain; therefore infer not X"
The fact that this argument scheme does not represent an invariably valid form is easily seen by a reductio ad absurdum: for there are many situations in which we know for certain neither X nor not X; but if argumentum ad ignorantiam were in all cases a valid argument form, then in such situations a first application of argumentum ad ignorantiam would produce conclusion not X, while a second application would produce conclusion X, from which we should deduce both X and not X, in violation of the fundamental reasoning principle of non-contradiction
Thus argumentum ad ignorantiam represents a argument scheme that cannot be regarded as invariably valid: in other words, argumentum ad ignorantiam is a logical fallacy
But careful thinkers should not conclude from the fact, that argumentum ad ignorantiam is a logical fallacy, that an appeal, to argumentum ad ignorantiam, always represents unsound reasoning:
Q: Why are you being so careful with that Luger?
A: Unless I know for certain that a gun is unloaded, I always assume it is loaded
Here the answerer, in practice, accepts not just any use of argumentum ad ignorantiam, but a particular use of argumentum ad ignorantiam
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