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Perception

Is there a way to perceive the real world? Thanks.

The obvious answer to your question is Yes. You perceive the real world by opening your eyes, listening, touching, etc. The real world is composed of trees and traffic lights and eagles, and you perceive them using your organs of sense. If you think this is a cop-out, that somehow the world I described isn't "the real world", then you'll have to say more about what "the real world" is and about why you think the world I described isn't really "real".
Perception

An old device called a stereopticon held two photographs taken from closely related viewpoints, such that on looking into it the observer saw a three-dimensional view of the photographed scene. This proves that we unconsciously construct, in our brains, a three-dimensional space out of two two-dimensional images, one per retina. Also, if you have someone hold up a finger, it is easy to bring your finger down on to its tip, but if you try this with one eye closed it is difficult -- proving that two eyes are necessary for seeing three-dimensional space. But this means that our three-dimensional visual space is inside our heads, whereas we clearly experience it as out side our heads. So which is it?

I see the laptop on my desk. This seeing entirely depends on some fabulously complicated neural shenanigans in my head. The three-dimensional laptop certainly isn't in my head. But what about my perception of the laptop -- is that in my head? I make a fist. Its existence entirely depends on the operation ofparts of my body. Is the fist inside my body or part of it, as my liver is? Nothing gets "constructed in your brain". This is a loose (if natural) way of speaking that can get you into trouble. If we open your brain, we will find nothing there beyond neural matter -- no images, no portions of visual space. The activity of your brain does make possible your apprehension of the world around you, but it's certainly not to be identified with what's apprehended. The three-dimensional world that I perceive does not exist in my brain, optic nerves, retinas, etc., even though it's true that I could not perceive it without the latter. In fact, I would go on to say that my perception of the...
Perception
Science

Is it true that in science 'theoretical' means 'non-empirical'? If so, are theoretical entities radically imperceptible? That is, although we can perceive the effects of theoretical entities, we can never perceive the entities themselves. For example, theoretical temperature is average kinetic energy of molecules, which we cannot perceive, but we can perceive its effects as thermometer readings and sensations of hot and cold; or mass is imperceptible but we can perceive its effects as forces of weight and inertia.

Sometimes, philosophers use the term "theoretical" to apply to certain statements in a scientific theory. Sometimes, they use the term to apply to certain entities whose existence is postulated by a theory, viz., those entities that are not directly observable. In the latter sense, they are contrasted not so much to "non-empirical" entities, but to observable ones. There is a lot of dispute about what "not directly observable" means. Are entities that we can see only with a telescope "directly observable"? Only with a microscope? Only with my glasses on? This kind of continuum has led some philosophers to declare that all entities are in principle observable. And others to hold that no entities are directly observable except "sense data", categorically unmediated sensory experiences. If you do think there are theoretical entities, or that most mature sciences contain statements with terms purporting to refer to them, then a major issue in the philosophy of science is what to make of (how...