 Three kinds of incompatibility are distinguished: mutual exclusion, contradiction and contrariety. Ideas \(A\), \(B\), C, … are said to exclude each other iff they are pairwise incompatible. The ideas \(A\) and \(B\) are contradictory iff \(B\) has “an extension that includes everything that does not stand under the idea \(A\)” (TS II, §103, 477), i.e. if \(B\) is equivalent to non-A. They are contrary, finally, iff they are incompatible but not contradictory.