I think there are three broadly principled and internally consistent epistemological stances: Radical skepticism, solipsism, and realism. Radical skepticism is principled because it simply demands extremely high standards before it will assent to any proposition; solipsism is principled because it combines skepticism with the Cartesian insight that I can be certain of my own experiences; and realism is principled because it tries to argue to the best explanation for phenomena in general, appealing to unexperienced posits that could plausibly generate the data at hand.

I do not tend to think so highly of idealistic and phenomenalistic views that fall somewhere in between solipsism and realism; these I think are not as pristine and principled as the above three views, and their uneven application of skepticism (e.g., doubting that mind-independent cucumbers exist but refusing to doubt that Platonic numbers or Other Minds exist) weakens their case considerably.