Exceptions are often cited as a counterargument against formal causation. Against this I argue that Aristotle explicitly allows for exceptions to essences in his biological writings, and that he has a means of explaining them through formal causation – though this means that he has to slightly elaborate on his general case theory from the Posterior Analytics, by supplementing it with a special case application in the biological writings. Specifically for Aristotle an essential predication need not be a universal predication. (...) Rather an essential predication is where a property belongs essentially to a species or genus. This essential predication serves a causal and explanatory role, and is not dependent on a corresponding universal predication. (shrink)