The Supreme Court has long refused to render formal advisory opinions, which is to say (quoting Black’s Law Dictionary), “nonbinding statement[s] … of [the Court’s] interpretation of the law on a matter submitted for that purpose.” Occasionally, majority opinions also say that they decline to opine on an issue that’s not squarely before them, because they don’t want to render an “advisory opinion.”
But it’s quite routine — and uncontroversial, at least among the Justices — for individual Justices, and groups of Justices not constituting a majority, to render “advisory opinions” in the sense of their own nonbinding statements of how they interpret the law. That’s one way that they signal to litigants which cases are potentially worth bringing in the future, or seek to dispel what they might see as a likely misunderstanding of the law among litigants or lower courts.
Indeed, many opinions are purely “advisory” in the sense of being nonbinding: all dissents qualify, as do concurrences to majority opinions. The same is true of dissents from the denial of certiorari, or statements respecting the denial of certiorari; in fact, the chief function of such opinions is to flag to the bar that at least one — and maybe several — Justices have a particular view on the law, and would grant certiorari in the right case. So a commenter on the Four Justices on the Fourth Amendment thread — which was triggered by a statement respecting the denial of certiorari — was possibly right in his first sentence when he wrote:
This seems to resemble an unsolicited advisory opinion to me. Does anyone know how often justices do this? I’ve never seen one before.
It is an unsolicited advisory opinion — just not of the sort that the Justices have generally condemned. And the answer to the question in the second sentence is “very often, in their concurrences and dissents, and several times a year in their dissents from denial of certiorari and statements respecting the denial of certiorari.”
Sometimes a Supreme Court majority opinion will contain language that discusses what the majority would do in cases not before the Court (and not just in the context of setting forth a general rule that’s applicable to the current case but also to others). And that sometimes is controversial, though many of the criticisms usually cast the matter in terms of the Court in engaging in “dictum” — often without the benefit of detailed adversary presentation — rather than in terms of “advisory opinions.” But nonbinding opinions by individual Justices, or groups of Justices short of a majority, have not aroused such controversy.