 Finally, many substance theorists, following Aristotle, have found it natural to distinguish necessary from essential properties, claiming that there are some necessary properties that are accidental. For example, everything necessarily has the property of being self-identical, but that property is a poor candidate as an essential property. A thing's essence provides its definition, answering the What is it? question. Being told that the thing is self-identical does not provide much by way of an answer to that question, as it does not offer grounds for distinguishing the thing in question from other things. Contrast this with the general essence of being human. Being told of some particular that it is a human does provide insight into what the thing is. And the property being identical to o is a good candidate for a singular essence. While the latter property does not so much ground the identity and diversity of o as simply presuppose it, it is a property cognitive grasp of which constitutes an understanding of what o is. 