In so many of his epistemological writings, Aristotle defends a sensible flavor of gradualism about our cognitive capacities: we Start with the partial grasps afforded by what is better known to us, and if things go well, we end up with understandings of those objects better known by nature. The picture is of a step-wise process, rather than a transforming moment of illumination.
In a difficult passage in Metaphysics IX, however, Aristotle introduces a kind of cognition which admits of no more or less, no better or worse. With respect to simples (ta asuntheta), Aristotle Claims, knowing is like touching — there is contact, or there isn't. Put differently, for simple objects, what is necessary for thinking of them at all is sufficient for grasping them completely. For this reason, Aristotle sees error about simples as impossible: any successful thinking about them will be such as to preclude error. The only possible mistakes are failures even to have them in mind.