In September 2016, Aaron Nevins, an obscure Florida political operative and blogger, sent an e-mail to Guccifer 2.0, the hacker that U.S. intelligence agencies believe to be affiliated with the Russian military. Guccifer, according to the Wall Street Journal, replied by handing Nevins a massive file of stolen e-mails from the Democratic Congressional Campaign Committee. The gleeful Nevins promptly shared the inside information on his blog and with reporters.

The ever-helpful Guccifer wanted to make sure that one person in particular was aware of Nevins’s post: Roger Stone, the mischievous informal adviser to candidate Donald Trump. Stone, who voluntarily posted his earlier text exchanges with Guccifer, says he didn’t share the hacked material with anyone. But if Stone had encouraged the hacker to push his ill-gotten information out into the public sphere, just what sort of violation might have occurred? Would it be, say, the digital equivalent of the Watergate break-in? Or the kind of oppo-dump that occurs in many a campaign, albeit with an unusual partner?

Maybe there are millions of rubles stashed in an unnumbered Swiss bank account to which only Paul Manafort knows the code words. Perhaps there’s an encrypted e-mail in which Russian ambassador Sergey Kislyak agrees to swap an anti-Hillary Facebook fake news barrage in exchange for President Donald Trump dropping Washington’s economic sanctions against Moscow.

The F.B.I. and special counsel Robert Mueller will keep looking for those and other, more subtle criminal acts. But in the current phase of the investigation, it’s become clear that investigators need to do more than prove or disprove collusion between Trump’s campaign and Russia. They need to define just what collusion is. Prosecutors and the public can agree on what bribery looks like when they see a stack of unmarked bills changing hands. But a shrug that encourages a hacker to root around in board of elections software is less easily traced and more easily denied. Will we know collusion when we see it?

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The House Intelligence Committee is trying to take the lead in exploring the collusion question by bringing in a series of high-profile witnesses. It wants to quiz Trump campaign digital director Brad Parscale, who reported to Jared Kushner. Did Parscale or anyone else on the social-media desk help the Russians target bots toward swing district voters? Parscale, in an interview with Fox News, said his campaign work did not intersect with Russian meddling efforts “in any way, shape, or form.”