On Tuesday, a federal judge ordered Apple to provide custom software that would help the FBI unlock an iPhone used by Syed Rizwan Farook and Tashfeen Malik, the couple who killed 14 people at a San Bernardino, Calif., office party in December.

The passcode on this particular phone—an older model—is protected by a feature known as 10-tries-and-wipe. Newer models store passcodes in so-called secure enclaves that Apple cannot crack.

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In this case, Apple could, in theory, give the FBI what it’s asking for, but the company has chosen to fight the court order. On Wednesday CEO Tim Cook posted an open letter on Apple’s aapl website explaining why.

“This moment calls for public discussion,” he writes.

So let’s discuss.

See also: Here’s Why Apple Is Going To War Over FBI ‘Backdoor’ Order.