“12 × 12 = 144”. Psychological Certainty versus Logical Exclusion of Error in On Certainty, Illustrated by the Example of Artihmetic. The aim of this paper is to exemplify Wittgenstein’s distinction between a psychological certainty and the logical exclusion of error by considering his discussion of our use of simple calculations like “12 × 12 =144” in On Certainty. Three aspects seem to be relevant in this context: 1. Wittgenstein’s notes on our use of the word “to know” in the context of these simple calculations (and on the irrelevance of inner states or processes for this use); 2. his discussion of the wish for “transcendental certainty” (OC 1975: 47); 3. his remarks on the logical exclusion of error.