I don't mean you *shouldn't*, I mean you literally can't. Here's why.
The issue is that when you go to make toast, with a toaster, you aren't exactly "making toast", in that the actual process is a bit more complicated, involving heating elements and chemical reactions and whatnot. Of course, you don't really need to understand; you just put bread into your metal box, wait a while, and then toast comes out. However, in that case, you don't really know how to make toast; you know how to use a toaster, but not how the toast is actually made. So, if you don't know what's going on inside your toaster, you don't know how to make toast.
Now, I know this sounds completely ridiculous, but bear with me; obviously, just because you don't know exactly how a toaster works doesn't mean you don't know how to make toast. However, that's because toasters are somewhat transparent in how they work; everyone knows that toasters have a heating element that toasts the bread, it's no secret. If, on the other hand, toasters were just big black boxes that took bread in one end and spat out toast on the other, then indeed, it wouldn't be reasonable to say you know how to make toast (unless you know what's going on inside that big black box, or how other toasters work). The notion of not having any idea how toast is made is obviously pretty ridiculous, but unfortunately it's the truth with many other modern technologies. The average person has no understanding of how their computer works, or how their car works, or how their air conditioning unit works, and so on.
This is great and all, but why does that mean I can't own anything proprietary? Sure, I don't have any idea how a car works, but I still own my car, right? Well, maybe not. Ownership implies that you have control over an object, but this is simply not the case with any of the examples I gave above. If you don't know how your car works, how can you possibly have any control over it? Realistically, you don't; you simply interact with it via an interface designed by the company who built it. You know how to use the interface, you own the interface, but you don't own the car itself.
So what, even if it *does* mean I don't own it by your definition, why should I care? Well, this is going to sound cheesy, but because knowledge is power. We are living in the so-called "Information Age", where information is without doubt the most valuable resource in our society. With this in mind, not only do you not own your [insert modern technology here], you are missing the most valuable component of it: the knowledge of how it works. Understanding how something works allows you to do far, far more with it than you could if you had no understanding whatsoever; you're able to use your possessions with their full potential, like nobody else could. They *truly* belong to you. I know it's hard to understand, and quite frankly hard to care, but never has it been more important to be aware of the world around you.
As an extra note, perhaps because of the fact that we're living in the Information Age, the use of information to exploit innocent people is becoming a more and more prominent issue in our society. No, your toaster isn't spying on you, but your car might, and your computer most definitely is. This is yet another example of how not understanding how something works means you have no ownership of it; without any understanding of its function, your computer behaves according to the will of the manufacturer, not according to *your* will. It's a vendor agent, not a user agent. Your iPhone never truly belonged to you; it always belonged to Apple.
This is why people care so much about open source. It isn't just some weird obsession, nor is it just about sticking it to the man and breaking away from capitalist industry; it's about having true ownership of your belongings. So, what should you do with this information? I challenge you to pursue open solutions whenever possible; choose open source software over proprietary alternatives when given the option, purchase your devices and tools from companies that don't try stop stop people from exercising their right to repair, or better yet, companies which publish detailed specifications of how their products work.
If you are still skeptical about what's been written here, I strongly recommend you talk to others about open source and related topics. This is a subject many people feel very strongly about, and a subject many people feel **must** be addressed if we ever hope to improve as a society.