A presumption in litigation is often described as an assumption the law makes, and the description misleads in two respects. A presumption is not an assumption about what is probably true, and it is not addressed to the parties. It is an instruction to the tribunal about what to do in a condition of uncertainty, and its operation is confined to that condition.
The mechanism is simple. A presumption identifies a fact -- that a letter properly addressed and posted was received, that a person absent and unheard of for a long period is no longer living, that a child born to a married woman is her husband's -- and directs that the fact be taken as established unless the party who disputes it produces evidence to the contrary. Where such evidence is produced the presumption performs no further function; the tribunal decides on the evidence, and the presumption has vanished from the case. It operates only in the interval before evidence arrives, and in the residual case where none does.
Its practical significance is therefore an allocation. The party who must displace a presumption bears the cost of assembling proof, and where proof is unobtainable that party loses. Every presumption accordingly embodies a judgement about which of two errors is preferable -- treating a letter as received when it was not, or as unreceived when it was -- and the judgement is made in advance, for a class of cases, by whoever formulated the rule.
That advance judgement is defensible where the classes are well chosen and the proof is genuinely easier for one side than the other. The postal presumption satisfies both conditions: the sender can seldom establish receipt and the recipient can readily establish non-receipt, so the burden falls where evidence is available. Suppose a club must give its members notice of a meeting, and one member says the notice never came. He may well be right, since letters do go astray. But he is the only person who can say what did and did not arrive at his own door, while the club can establish only that it addressed the envelope correctly and paid the postage. If the club had to prove receipt it would lose every case of this kind, including the occasions on which the letter plainly arrived. If the member has to prove non-receipt he must say so on oath and give his account of the matter, which is a thing he can do. Presumptions that fail one of the conditions are objectionable for that reason and not because they assume anything.
The persistent misdescription therefore matters. Criticism directed at a presumption as an assumption invites the reply that the assumption is usually correct, which is true and irrelevant. The question is never whether the presumed fact is likely. It is whether the party required to disprove it is the party better placed to do so.
