 He then goes on to reformulate the definition for reasons related to his view that only actually produced sentences (sentence-tokens) can have a truth-value (Klima 2004; Dutilh Novaes 2005). ‘No sentence is negative, therefore no donkey is running’ comes out as a valid consequence according to the criterion thus formulated, because ‘No sentence is negative’ can never be true: its mere existence falsifies itself whenever it is produced. According to Buridan, this example should not count as a valid consequence, and one reason he gives for this is that its contrapositive ‘Some donkey is running, therefore some sentence is negative’ is not a valid consequence. He formulates a definition of consequences in terms of ‘howsoever the antecedent/consequent signifies things to be’ in order to accommodate such counterexamples, but adds that in most cases, the simpler definition is sufficiently accurate.