 Propositions concerning relations of ideas are intuitively or demonstratively certain. They are known a priori—discoverable independently of experience by “the mere operation of thought”, so their truth doesn’t depend on anything actually existing (EHU 4.1.1/25). That the interior angles of a Euclidean triangle sum to 180 degrees is true whether or not there are any Euclidean triangles to be found in nature. Denying that proposition is a contradiction, just as it is contradictory to say that 8×7=57.