It will be convenient to begin by restating a few basic principles relating to the concept of possession under English law:
(1) In the absence of evidence to the contrary, the owner of land with the paper title is deemed to be in possession of the land, as being the person with the prima facie right to possession. The law will thus, without reluctance, ascribe possession either to the paper owner or to persons who can establish a title as claiming through the paper owner.
(2) If the law is to attribute possession of land to a person who can establish no paper title to possession, he must be shown to have both factual possession and the requisite intention to possess (animus possidendi).
(3) Factual possession signifies an appropriate degree of physical control. It must be a single and conclusive possession, though there can be a single possession exercised by or on behalf of several persons jointly. Thus an owner of land and a person intruding on that land without his consent cannot both be in possession of the land at the same time. The question what acts constitute a sufficient degree of exclusive physical control must depend on the circumstances, in particular the nature of the land and the manner in which land of that nature is commonly used or enjoyed ... It is impossible to generalise with any precision as to what acts will or will not suffice to evidence factual possession ... Everything must depend on the particular circumstances, but broadly I think what must be shown as constituting factual possession is that [t]he alleged possessor has been dealing with the land in question as an occupying owner might have been expected to deal with it and that no-one else has done so.
(4) The animus possidendi, which is also necessary to constitute possession, ... involves the intention, in one's own name and on one's own behalf, to exclude the world at large, including the owner with the paper title if he be not himself the possessor, so far as is reasonably practicable and so far as the processes of the law will allow ... the courts will, in my judgment, require clear and affirmative evidence that the trespasser, claiming that he has acquired possession, not only had the requisite intention to possess, but made such intention clear to the world. If his acts are open to more than one interpretation and he has not made it perfectly plain to the world at large by his actions or words that he has intended to exclude the owner as best he can, the courts will treat him as not having had the requisite animus possidendi and consequently as not having dispossessed the owner.