Our entire world is a construct of the brain as interpreted through signals received from the sensory organs. There is no such thing as "bright" or "hot" or even "color" except in subjective experience, so why couldn't this be true of everything else too?
Many philosophers and epistemologists agree that reaching "absolute certainty" in terms of our knowledge of anything is impossible. We can't even be absolutely certain of our own existence (we could be in the matrix or a brain in a vat). We can, however, be MAXIMALLY certain.
certainty is determined by reality, which is determined by concentration and knowledge of each individual, thus, we can know something for certain if we establish the term "certainty" as a fact that is temporary provable, but also discredited.
Human fallacy does not prove you cannot know anything for certain. For instance, humans could create an artificial intelligence without these imperfections. If they did so, this conjecture still applies to them with equal validity.