Aristotle’s argument in Physics II 8 can be summarized as follows: Dogs typically develop teeth good for biting and chewing. A typical result is not a coincidence. So it’s not a coincidence that dogs develop teeth good for biting and chewing. If the development is not coincidental, it must be “for something”. So the dog’s development is “for something”. (that is, it is goal-directed) The problem with this argument lies in premise 4.