Thus (e.g.) he who has said that breathable is a property of air has rendered the property potentially (for that is breathable which is such as can be breathed), and he has also rendered the property relatively to what does not exist (for while air may exist, even though there exists no animal so constituted as to breathe the air, it is not possible to breathe it if no animal exists; so that it will not be a property of air to be such as can be breathed at a time when there exists no animal such as to breathe it)