Finally, some presumptions, such as res ipsa loquitur, seem to be devices by which we “smoke out” the opposing party (usually the defendant) to tell us what he knows about the event. These presumptions were first applied when there was little or no discovery, and a defendant, merely by stonewalling, could effectively prevent the plaintiff from proving his case. Byrne v. Boadle, 159 Eng. Rep. 299 (1863).
We usually speak of this process as presuming fact B (delivery of the letter) from the basic (or predicate) fact A (posting of the letter), and require that there be some connection between facts A and B. This may be graphically illustrated as follows:
(Predicate) A → B (Presumed)
Thus, if the jury finds fact A by the proper standard, it may conclude that the presumed fact (B) is also proved.
There is considerable uncertainty, even in civil cases, as to the procedural importance of presumptions. Although presumptions based on policy decisions, such as the child-father rule mentioned above, are irrebuttable, most presumptions are rebuttable. The question contested is, who has to rebut, and to what degree. Some courts and writers argue that a presumption, at least one based on common experience, should always shift the burden of (dis)proof to the opposing party (whom we will refer to as the defendant, since it usually the plaintiff who seeks to use a presumption). E. Morgan, Basic Problems of Evidence (1963). Others argue that most presumptions are simply “smoking out” devices and should disappear entirely if the defendant comes forward with as much evidence as he has. J. Thayer, A Preliminary Treatise on Evidence at the Common Law (1898).
These rules may be more easily understood as they are applied. If you see puddles in the street after you’ve been in a building for hours, you are likely to conclude that it has rained, although you didn’t see it rain. Why? Because “in the vast majority of cases” puddles in the streets come from rain. A presumption that “puddles on the street implies rain” is probably commonsensical: Proof of the predicate fact A (puddles) leads to the conclusion B (that it has rained). You may later learn that the water was from some other source (e.g., a street cleaner or an overturned water truck), but you start from the premise, based on common experience, that if there are puddles, it is highly likely that it rained. Indeed, in the absence of other suggestions, you are likely to conclude beyond a reasonable doubt that it rained.
These same empirical considerations may apply to criminal cases. Suppose, for example, that statutes prohibit the possession of certain drugs only if they have been imported into the United States. An instruction to the jury that if the prosecution proves the drug to be heroin, it can be presumed that it was imported, unless the defendant brings some evidence to the contrary, is constitutional because virtually no heroin is produced in the United States. On the other hand, that same instruction applied to marijuana is probably invalid because much marijuana (even if not over 50 percent) is homegrown. See Leary v. United States, 395 U.S. 6 (1969).
In earlier centuries, the criminal law employed many such presumptions. Thus, a defendant was “presumed” sane. Similarly, a person who used a deadly weapon in killing another was “presumed” to have “malice aforethought” (or, in a variation of this presumption, to “intend” the death). Even more broadly, defendants were said to be “presumed” to “intend the natural and probable consequences of their acts.” Some of these presumptions were established not only because they might be commonsensical, but also because defendants were precluded from testifying. Thus, mens rea “had to be” presumed from facts proved by the prosecution. Whether these presumptions are valid today, when defendants have a constitutional right not to testify and not to have their silence construed against them, is highly doubtful.