The ability to identify arrangements affecting property as falling within one or other of the recognised categories of proprietary interest, for example tenancies, licences or easements, and as resulting in certain consequences, is not only desirable, but is in the interest of certainty for all concerned essential. If an occupant of land is a tenant, certain known consequences follow: for example, the occupant has statutory protection which would not be the case were he merely a licensee. Again, if I cross your land merely by your permission, my privilege may be withdrawn at any time, whereas if I have an easement to do so, you cannot stop me. Arrangements which do not behave according to the rules, by appearing to create one type of interest while possessing the attributes of another, either undermine the recognised rules or are exceptional cases which must be acknowledged as such.