Yes. Maths. 1+1=2 is 100% accurate and you can prove it by taking an apple, taking another apple and ending up with two. Apart from that you can't be 100% sure about anything, there are so many conspiracy theories and contradictory opinions about everything. read more
If to know something with certainty means having undoubtable, true thoughts, the answer is: We cannot even determine for certain whether we know anything about the world [i.e. anything that we learn through our senses], but we can know the form of our thinking (and sensing, and other faculties) for certain. read more