A property is something which does not indicate the essence of a thing, but yet belongs to that thing alone, and is predicated convertibly of it. Thus it is a property of man to be capable of learning grammar- for if he is a man, then he is capable of learning grammar, and if he is capable of learning grammar, he is a man. For no one calls anything a property which may possibly belong to something else, e.g. sleep in the case of man, even though at a certain time it may happen to belong to him alone. That is to say, if any such thing were actually to be called a property, it will be called not a property absolutely, but a temporary or a relative property- for being on the right hand side is a temporary property, while two-footed is a relative property- e.g. it is a property of man relatively to a horse and a dog. That nothing which may belong to anything else is a convertible predicate is clear- for it does not necessarily follow that if something is asleep it is a man.
A property does not describe what something is, but it is only true of that thing and it is always true of that thing. For example, it is a property of humanity to be capable of learning grammar. All human beings are capable of learning grammar and only human beings are capable of learning grammar. Properties cannot even possibly belong to something else. For example, capable of sleep is true of sheep as well as humans and is not a property of either one. Suppose that you picked out a human being by saying that sleeping creature. You would not be pointing out a property of human beings even if every other creature was awake. You could call such a thing a temporary property, because for a short time it did pick out only human beings. Likewise, you could call being two-footed a relative property in comparison with horses and dogs since they have four feet. Temporary and relative properties are not really properties though because they fail to pick out what is true always and only of that thing under all possible circumstances. Simply being asleep does not mean that something is human, even if that human is the only sleeping creature right now on the entire planet.
Once again, there is a problem comparing Aristotles definition of property with our contemporary definition. In contemporary definitions, we call accidents properties. We also call definitions properties. We even call true statements about something properties of them. I am going to avoid that here and use only the definition given by Aristotle. I also mentioned that I would explain more about properties and definitions. A brief statement about the difference is that we can determine from a definition whether or not a property is true of it. But properties cannot always tell us whether or not another property is true of that thing.
Next, Aristotle will describe what he means by genus.