Facebook is building tech to read your mind. The ethical implications are staggering.

Facebook wants to create a device that can read your mind -- literally. It's funding research on brain-machine interfaces that can pick up thoughts directly from your neurons and translate them into words, the company announced in a blog post last week. The short-term goal is to help patients with paralysis, by decoding their brain signals and allowing them to "speak" their thoughts without ever having to move a muscle. That could be a real public good, significantly improving quality of life for millions of people. In the US alone, 5.4 million people currently live with paralysis.