It seems like biometric-enabled gadgets should be a hard sell in the post-Snowden marketplace. When Apple announced the Touch ID fingerprint sensor on the iPhone 5s, Twitter lit up with a blaze of NSA jokes. And yet Apple sold some 4 million 5s’s in the first weekend.

This is no surprise, of course. History suggests we’ll learn to accept any new and cool technology the industry throws at us, no matter how initially creepy. In all likelihood, our devices will soon read our prints and recognize our voices and irises. “The integration of biometrics in consumer electronics is really a forgone conclusion,” says Steve Koenig, director of industry analysis for the Consumer Electronics Association.

But I have a theory: If—if—consumers ever draw a line, they will do so when their gadgets start tracking their eyes.

Psychologists and other scientists have long used eye-tracking cameras for research, but the technology is just beginning to move into consumer products. The company MindFlash is beta-testing FocusAssist, an eye-tracking feature that, paired with corporate-training software, can let Human Resources know whether you really paid attention to that orientation video. Tobii Technology, a Swedish firm, is developing eye-tracking technology for use in laptops. The new Samsung GS4 smartphone pauses videos when you look away (although this isn’t true eye-tracking technology; the phone does this by tracking your face and eyes). Google has patented gaze-tracking technology that could allow it to charge advertisers literally by the eyeball.

To my mind, these technologies are categorically different from, say, Apple’s Touch ID. Touch ID is designed to increase security. (Set aside the question of whether it works; within days of the iPhone announcement, German hackers announced that they had cracked Touch ID with a fingerprint they lifted from a piece of glass.) Trust Apple with your fingerprint, the idea goes, and Apple can better protect your bank information.

But how important is it that your phone pause videos when you look away? Does the benefit outweigh the potential loss of privacy?

There is something intrinsically creepy about a device—manufactured by a multinational corporation that stands to profit by gathering intimate information about you—that continuously monitors the movements of your eyeballs.

Here’s how eye-tracking works: A diode shines near-infrared light on your eyeball, while a camera continuously takes photos of your eye. “It’s looking for two pieces of information: the shape and the orientation of the pupil, and reflectance from the cornea,” says Michael Hout, a visual cognition researcher at New Mexico State University. A processor then builds a 3D representation of your eye. “If it can figure out how much your eye is rotated, it can figure out where your eye is pointed.” Thus, it can figure out what you are paying attention to.

You can’t feel the infrared light, of course. The whole process is imperceptible. So, in a sense, is the motion of your eyes. “You’re not thinking about where you’re looking—it just comes naturally,” Hout says. “Eye movements are outside of the influence of someone’s overt thought.”

The preconscious nature of this whole process is what I suspect could give people the willies in a way that fingerprint sensors do not. An eye-tracking gadget knows where you’re looking before you know yourself. It seems to have the ability to spy on the interior of your consciousness.

Is there any evidence that people will reject eye tracking? Not a lot, other than snarky reactions online to MindFlash’s Focus Assist. And like I said, it is possible—even likely—that electronics manufacturers will introduce biometrics in such measured, shiny doses that we will gradually become comfortable with them.

Still, I have to think that at some point consumers will draw a line. It might not come until Google asks us all to send in a vial of blood for their Google+ DNA bank. But if it happens with eye tracking, you heard it here first.