Klein is widely known for his work on skepticism. His most influential work, however, is on the nature of knowledge, where he has long defended the defeasibility theory. His recent work defends infinitism about justification. On this view, to be justified in believing P is to possess a reason R1 to believe P, and a reason R2 to believe R1, and a reason R3.....and so on, ad infinitum. Justification is, so to speak, "turtles all the way down." He has also recently advocated a picture of knowledge according to which one can have knowledge of p even if the justification for the belief that p is essentially based on false premises. Klein calls these "useful falsehoods".