Yang: Yeast has long been known to be a leaven, that is, a substance used in baking to make breads rise  Since biblical evidence ties the use of leavens to events dating back to 1200 B  C  , we can infer that yeast was already known to be a leaven at that time  Campisi: I find your inference unconvincing; several leavens other than yeast could have been known in 1200 B
C
pointing out that the premises of Yang's argument more strongly support a contrary conclusion