If one is remarkably confident that one can, indeed, observe things, then this is a dilemma. To such a person, this word, "observe," has a very clear crisp meaning that could not possibly ever be challenged.
They can use that confidence to be sure of logical or metaphysical truths.
Of course, those observations are never deceiving:
Actually, if you study empiricism, you find that it is a subcategory of epistemology, the study of what we can "know." Empiricists believe that all knowledge stems from "sensory observations," though what those are is a heavily debated topic. Another branch of epistomology, rationalism, believes that all knowledge stems from the use of reason independent from the senses. In rationalism, it is logical or metaphysical truths that one can be certain of, and the "observable" world outside is the part which is treated with suspicion.