Unlawful - Innocent Possession
Possession of a restricted or unlawful item is a potential crime in every legal jurisdiction in the world. Whether that item is a weapon, drugs, stolen property or even contraband cigarettes, in Ontario at least, both the actus rea (guilty act) and mens rea (guilty mind) must be proven for there to be valid conviction. That means that not only must the defendant be in ‘possession’ but they must know they are in such possession.
Possession as defined by the Criminal Code of Canada
4. (3) For the purposes of this Act,
(a) a person has anything in possession when he has it in his personal possession or knowingly
(i) has it in the actual possession or custody of another person, or
(ii) has it in any place, whether or not that place belongs to or is occupied by him, for the use or benefit of himself or of another person; and
(b) where one of two or more persons, with the knowledge and consent of the rest, has anything in his custody or possession, it shall be deemed to be in the custody and possession of each and all of them.
This implies three types of possession discussed here; actual, constructive and joint possession. 14 Section 4(3) of the Code creates three types of possession:
(i) personal possession as outlined in section 4(3)(a);
(ii) constructive possession as set out in section 4(3)(a)(i) and section 4(3)(a)(ii); and
(iii) joint possession as defined in section 4(3)(b).
This attempts to cover a plethora of circumstances and leaves a great deal of discretion to the charging authority to define the situation such as to bring it within the definition. Still, the burden of proof always rests with the crown to prove beyond reasonable doubt that there was actual possession.
However, circumstantial evidence of possession may well be enough. In R. v. Pham  Kozak J. of the Court of Appeal said “The circumstantial evidence supported as the only logical inference a consistent awareness of, and participation in, all that occurred in her home on the part of the accused, and demonstrated much more than a quiescent or passive knowledge of the drugs, as well as an element of control over them.” In R. v. Chalk  the court said “His instruction to delete the material was a manifestation of his longstanding power or authority over the material. He had "possession" of the child pornography.”
The same case also made it clear that possession can occur even if the item is ‘owned’ by a third. So, keeping knowingly allowing drugs to be stored at your residence can be construed as possession as long as the person has a) knowledge of the object, b) consent of the accused and a degree of control over the item.
Apparently, possession for the purpose of destroying the item is not criminal so a parent taking illegal drugs from a child in their care is not possession at law. See R. v. Glushek , R. v. Christie , R. v. York , R. v. Loukas, .
The court can ‘construct’ the element of guilt from the totality of the evidence given as in R. v. Sparling  the court said "There is no direct evidence of the applicant’s knowledge of the presence of narcotics in the residence. It is not essential that there be such evidence for as with any other issue of fact in a criminal proceeding, it may be established by circumstantial evidence. In combination, the finding of narcotics in plain view in the common areas of the residence, the presence of a scale in a bedroom apparently occupied by the applicant, and; the applicants apparent occupation of the premises may serve to found an inference of the requisite knowledge"
While it is true that the crown must prove every element of the offence, the determination of possession is decided by looking at the evidence as a whole. R. v. Brar  ‘Knowledge’ implies that the accused knew the criminal nature of the item.
Retaining possession after discovery of an item can be determined as possession as in R. v. Christie , the defendant was found guilty when she held onto discovered drugs in car for even an hour.
The crown must prove that the person has the power or ability to exercise some of control and not that they actually exercise control. In R v Wu  three adults lived in a house with a grow-op on the upper floor but with no evidence of active involvement were still deemed to be ‘in possession’.
Constructive possession may be proven by an attempt to conceal discovery of an item. In R. v. Lou Hay Hung  "where one of two persons has opium in his custody or possession, another who knows that fact, even though he has no measure of control over it, but nevertheless co operates with the person who has such custody in an effort to prevent detection" that person has possession.