Why use "can and may" both in a sentence?

Question

I have seen a lot of questions about the difference between can and may and I am aware of them. In a legal(-ish) document (some policy) I have read a statement to the effect of a consequence can and may include a list of possibilities. In any everyday context that I can imagine either one of them (can or may) would have sufficed and expressed pretty much the same sentiment. However, I am neither a legal expert nor an English native speaker.

My question is, why does this sentence need to specify that the result of some action can and may include some consequences?

The reason may be a legal distinction that is not within the scope of this community. However, I wanted to try the linguistic approach first.

A good answer would also provide an example of when an action can but may not include a consequence or vice versa.

For context, you can and may ;) use an example like the following (really only an example!):

Failure to submit your homework assignment on time allows your teacher to take appropriate actions. Such actions can and may include a reduction of your grade, exclusion from the final exam, or a written note to your supervisor.

Actually, while re-reading my question, I came to the conclusion that writing "can and may" both may mostly serve as affirmation or emphasis. If the document only states that the actions can include the listed consequences, a pedantic reader might ask if that would be legally allowed. If the document only states that the actions may include some consequence, a pedantic reader might ask if those consequences are technically even possible. Although the latter seems less probable: in the example above, if the teacher may reduce your grade there is no reason to doubt that they actually can do that. Similarly, if they are permitted to exclude you from the exam or write to your supervisor, there is very little that could prevent them from having that power. In any case, by writing can and may, even a pedantic reader would need to be very motivated to double-check the correctness of that statement.

Still, I would love to get an expert opinion on this or maybe a little background. Since my answer is pure speculation, I doubt it would meet the quality criteria on this site. There may very well also be fine points that I just don't know about.

So, please elaborate on why a legal document would or even should consider using both - can and may - in the context of consequences of certain actions or failure to meet standards et cetera.

Answer 1

Short Answer

Although there is a subtle difference in meaning between can and may in these contexts (explained below), may implies can, so including can in such formulations is redundant. If one sees both, that is probably because the drafter was keen to ensure against any possibility of misunderstanding, without thinking too much about what that possibility might be. (Legal drafting often includes redundancies to ensure against even very unlikely misunderstandings, because a misunderstanding can be harmful, while redundancies are generally harmless.)

Constitutive and Regulative Rules

Some rules that one finds in the law, as well as in many other systems of rules (including the rules of schools, private clubs, etc.), are constitutive: they partially define, constitute an institution. For example, the institution of legal marriage is partially defined, constituted by the rule that only certain officials can pronounce two people married. If somebody else were to utter the same words, that wouldn’t count as a wedding. In other words, only certain officials have the legal power to perform weddings; it is legally impossible for somebody else to do it.

Other rules, such as the rules prohibiting assault, theft, etc., are regulative: they regulate independently existing activities. When these rules are violated, some wrong has been committed, and some kind of a sanction is called for. Note that the phrases doesn’t count as, and legally impossible, which are apt in characterising something that fails to fit constitutive rules, have no place in describing the violations of regulative rules.

The terms constitutive and regulative for these two kinds of rules were introduced by the mid-twentieth-century philosophers of language (the best known exposition of the distinction in these terms appears in the context of John Searle's theory of speech acts), but arguably equivalent distinctions had been made, using other terms, in jurisprudence well before that time, most notably in the theory of Wesley Hohfeld, early in the twentieth century.

Now, the word can (in the sense that is relevant here) can be used to express constitutive rules: saying that such-and-such officials can perform weddings is another way of saying that they have the legal power to do so. Saying that others cannot perform weddings is another way of saying that it is legally impossible for them to do so. Or to use the OP’s own example, saying that the teachers can reduce the students' grades in the specified cases means that they are empowered by the rules of their educational institution to do so, that the rules make it possible for them to do it.

On the other hand, may is used for expressing regulative rules: it means that something is permitted by these rules, that there is no punishment for doing it. Some regulative rules stand on their own, but some are superimposed upon constitutive rules. In the OP’s example, the teachers not only (1) have the power, bestowed on them by a constitutive rule, to reduce the grades, but (2) there would also be nothing wrong with their exercising that power (i.e. there is no regulative rule against exercising it). In the can and may locution that the question is about, can is used for (1), while may is used for (2). May expresses what is in Hohfeld's, somewhat confusing, terminology a privilege (what some other theoreticians call a liberty), while can expresses what is in his terminology a power.

However, the question of whether somebody may do something would not arise if it were impossible to do it; that’s why may implies can, and makes it redundant (even though can does not imply may).

Alternative Interpretation

Alternatively, one can interpret may in can and may as a prediction of sorts, rather than an expression of a rule, while interpreting can the same way as above. On such a reading of the OP's example, (1) the teachers have the power to reduce the grades, and (2) the probability of their exercising that power is sufficiently high that it should not be ignored. As Mr Ashworth has, more colloquially, put it in the comments, the OP's example can be interpreted to mean 'teachers are entitled to reduce your grade and don't think they never use this power'.

This interpretation still leads to the same verdict as the one offered above: may implies can, and makes it redundant (because the question on how probable something is would not arise if it were not possible in the first place).

Answer 2

The remit of this site is not discussion of the motivation of writers but the meaning of their words. We leave the reader to speculate on their motivation.

As a simplification, and in the context of the question, one can differentiate between “can” and “may” as follows:

1. “Something can happen” means that it is possible according to the laws of physics (or in our experience of the world).

2. “Something may happen” means that it is possible according to the laws of society (or a social institution).

[See e.g. Merriam-Webster]

So I would interpret this as a combination of (1) a warning, and (2) a “legal” statement:

1. If you do this it is possible that your exam will be returned with a lower grade.
2. If the marker does lower your grade his action will be perfectly legitimate within our regulations.