In his congressional testimony, he illustrates the difference between regulating activity and inactivity:

Here's a thought experiment. Imagine that I tell you 100 things that you may not do tomorrow. For example, you cannot run on a treadmill, eat broccoli, buy a car, and 97 other things. While your liberty would be restricted, there would still be an infinite number of things you may still do.
Now suppose I tell you 100 things that you must do tomorrow. You must run on a treadmill, eat broccoli, buy a car, and 97 other things. These 100 mandates could potentially occupy all your time and consume all your financial resources.

And also, of course, the difference between negative and positive rights.