Abstract
I criticize and emend J L Mackie's account of causal priority by replacing ‘fixity’ in its central clause by 'x is a causal condition of y, but y is not a causal condition of x'. This replacement works only if 'is a causal condition of' is not a symmetric relation. Even apart from our desire to account for causal priority, it is desirable to have an account of nonsymmetric conditionship. Truth, for example, is a condition of knowledge, but knowledge is not a condition of truth. My definitions of 'sufficient condition for' and 'necessary condition for' do not imply that p is a sufficient condition of q if and only if q is a necessary condition of p.