Special counsel Robert Mueller’s assessment of Donald Trump’s efforts to obstruct justice hinged in part on one key question: whether the president can even commit obstruction of justice in the first place. Trump’s defenders, including William Barr (before he became Trump’s attorney general and personal bodyguard), consistently claimed that prosecutors had no business assessing the president’s motives in exercising his sweeping constitutional powers. To commit obstruction, Trump had to act with “corrupt intent.” But does the special counsel have any authority to evaluate the president’s actions for unlawful intent if he can justify them with some neutral pretext?

Yes, Mueller answered emphatically in his report. To do so, he deployed various constitutional and statutory analysis—including, notably, a citation to an opinion by Supreme Court Justice Antonin Scalia. That opinion helps illuminate the legal assumptions that underpinned Mueller’s suggestion that Trump obstructed justice, as well as his argument that the Justice Department could in fact investigate a sitting president for the crime of obstruction.

There are three elements in the federal crime of obstruction: an obstructive act, some kind of nexus between that act and an official proceeding (current or pending), and corrupt intent. This last element, corrupt intent, posed a problem for Mueller. Many actions that the special counsel assessed as possible evidence of obstruction were, on their face, legal. For instance, under the Constitution, the president clearly has the power to fire a subordinate in the executive branch, so his termination of FBI Director James Comey was not obviously unlawful. But what if Trump fired Comey for unscrupulous reasons? Specifically, what if he dismissed Comey in an effort to stymie the investigation into his conduct during the 2106 election, as Trump himself later implied? In that case, could Trump’s “corrupt intent” render his action unlawful obstruction?

Yes, Mueller concluded, citing, among other things, Scalia’s 1995 opinion in United States v. Aguilar. That case involved the conviction of a federal judge for illegally disclosing a secret wiretap then lying to FBI agents about his leak. Aguilar turned, in part, on the definition of “corruptly,” and Scalia wrote that the word “has a long standing and well accepted meaning” in criminal law. Quoting from law dictionary and precedent, Scalia wrote that “corruptly” denotes an “act done with an intent to give some advantage inconsistent with official duty and the rights of others.” (Emphasis mine.) Mueller then wrote:

That standard parallels the President’s constitutional obligation to ensure the faithful execution of the laws. And virtually everything that the President does in the routine conduct of office will have a clear governmental purpose and will not be contrary to his official duty.

Thus, only in truly exceptional cases will prosecutors be able to glean corrupt intent behind seemingly proper or “routine conduct of office.” And the special counsel’s decision to gauge Trump’s intent will not chill the president’s actions, because “in virtually all instances, there will be no credible basis for suspecting a corrupt personal motive.”

Of all the cases Mueller could cite to make this point, why the Scalia opinion in Aguilar? It is not actually an opinion for the court but rather a partial dissent, not a binding authority. But this citation serves two useful purposes. First, it clarifies that in federal criminal law, individuals, including government officials, can engage in obstruction when they commit a seemingly lawful act with illicit motive. After all, the law contemplates an individual having some “official duty” with which he acts inconsistently. If, for example, the president purports to exercise his Article II authority to terminate unsatisfactory subordinates, but actually fires an underlying to impede an investigation into his campaign, he may have acted “inconsistent with official duty” and therefore with the necessary corruption to obstruct justice.

Second, by bolstering his argument with wise words from Scalia, Mueller preemptively neutralized some inevitable criticism that his report failed to respect the presidency. Scalia was a proponent of the “unitary executive” theory, which endorses vast powers for the president. And the famed conservative jurist is a model for Trump’s archconservative judicial nominees. Yet Scalia’s Aguilar opinion seems to fortify Mueller’s rationale for investigating Trump’s ostensibly officials acts, forestalling complaints that the special counsel’s obstruction probe chilled the president’s freedom to govern.

In the end, Mueller concluded that he did not gather sufficient evidence to determine whether a “reasonable inference” could show that Trump fired Comey with corrupt intent. He did, however, write that a number of Trump’s other actions, including his efforts to manipulate then–Attorney General Jeff Sessions, arguably qualified as obstructing justice. If Trump had untrammeled authority to boss around his subordinates, regardless of intent, Mueller’s inquiries into these episodes would’ve been pointless. But as Scalia seemed to recognize, an official act can still carry a corrupt intent when it is performed “inconsistent with an official duty.” And Trump had a duty to execute the laws—not protect himself and his associates from investigation.