An accident is something which, though it is none of the foregoing i.e. neither a definition nor a property nor a genus yet belongs to the thing- and something which may either belong or not belong to any one and the self-same thing, as (e.g.) being seated may belong or not belong to some self-same thing. Likewise also whiteness- for there is nothing to prevent the same thing being at one time white and at another not white. Of the definitions of accident the second is the better- for in the case of the first, any one is bound, if he is to understand it, to know already what definition and genus and property are, whereas the second is sufficient of itself to tell us the essential nature of the thing in question. To accident are to be attached also all comparisons of things together, when expressed in language that is derived in any kind of way from accident- such as, for example, the question, Is the honourable or the expedient preferable? and Is the life of virtue or the life of self-indulgence the pleasanter?, and any other problem which may happen to be phrased in terms like these. For in all such cases the question is of which of the two is the predicate more properly an accident? It is clear on the face of it that there is nothing to prevent an accident from becoming a temporary or a relative property. Thus being seated is an accident, but will be a temporary property, whenever a man is the only person sitting, while if he is not the only one sitting, it is still a property relatively to those who are not sitting. So then, there is nothing to prevent an accident from becoming both a relative and a temporary property- but a property absolutely it will never be.
An accident is something that of true of a thing but is not a definition, property or genus. We could also define an accident as something that is true of a thing but could have not been true of it. For example, being seated may be true of a thing now but not later and being white may be true of a thing later but not now. The second definition of an accident is better than the first one because the first one assumes that you already know what definitions, properties and genus are. Any discussions involving any form of comparison are discussions about accidents, for example, Is the life of virtue or the life of self-indulgence more pleasant? In that question we are asking which one of them pleasant applies in a stronger way to. That is just a question of which accident another accident is truer of. This is true for all comparisons. Aristotle mentioned temporary and relative properties in his discussion on properties. Here he mentions that accidents can become temporary and relative properties. Whether something is an accident or a relative or temporary property depends on something other than that thing. That is why no accident is a property.
Aristotle concludes this discussion without an explicit proof that these are the only kinds of arguments that it is possible to make. Nonetheless, such an argument is possible, and Aristotle likely thought that it was too obvious to mention. Since every question is about something, and everything true of something is either a definition, property, genus or accident, every topic will be divided without remainder into those four categories.
Next, Aristotle concludes his discussion of the various topics by dividing the topic of dialectic into parts.