This paper discusses the issue of whether the power (dynamis) is a criterion of being, or a definition of being at Plato's Sophist 247d8-e4. I propose a new solution to this problem in light of the Hippocratic Method Passage at Plato's Phaedrus 270d, arguing that, when one takes this parallel passage into account seriously, the dynamis proposal at Sophist 247d8-e4 only provides a criterion of being. This paper first gives some preliminary remarks on Plato's discussion of the notions of 'definition' and of 'criterion'. Then I argue that the Phaedrus passage is a proper parallel passage of the dynamis proposal passage. After establishing the understanding of physis as essence in both the Hippocratic Corpus and Plato's dialogues, this paper provides a detailed textual analysis between the Hippocratic Method Passage and the dynamis proposal passage. I demonstrate that the dynamis proposal at most provides a necessary condition of being, which falls short of being a proper definition of being. Therefore, the dynamis proposal only points to a criterion of being.
