Do Hallucinations Complicate the Veracity of Knowledge?

A facebook conversation: if we can hallucinate, how do we know anything is true?

Hallucinations concern particular contingent things or sensory events. Likewise, erroneous intellectual judgements mistake some special or categorical aspect of reality. In both cases there are multiple potential "sufficient reasons" or causes for the state of affairs concerning which we are mistaken. This is also the case in example you cite. There can be no certainly that something occurred because you prayed for it (for perhaps some other reason or chain of reasons brought it about).

Meanwhile thomsitic treatments of being & existence of God concern "transcendental" or transcategorical realities. We can be mistaken concerning the veracity of a sense impression (maybe it is a hallucination). But whether we are hallucinating, in the matrix, brains in vats etc, we are beings. The particular mode at stake is utterly irrelevant and conversation abstracts from it. If our being is participated or externally actuated (distinct from our essence as act to potency), we will exhibit a causal dependence on being-itself or God. There is nothing extrinsic to being and only a single "sufficient reason" for it (pure act). Skepticism as is possible concerning sense impression takes for granted some partial and contingent thing (or "dualism" as hegel says: a distinction between the object under consideration & possibilities/states/realities extrinsic to it).

This is why thomists say "exercised radical doubt" is impossible: the gesture of skepticism can concern only particular modes of being.