There are schools of thought for which there is no such thing as reality. Everything is simply an illusion or a social construction. If it is an illusion, though, it is such a persistent one over time and between people that, for all practical purposes, reality is real.
Every effect has at least one cause, even if we don’t know all or any of them. Every cause has an effect, even if the effect is imperceptible. By taking a breath, you cause certain measurable changes within your body and to the external environment. Every action you take has some effect, large or small. Modern civilizations have brought nature under control to some degree and have improved the lot of billions of people by discovering causes and effects for millions of different processes, because reality is persistent enough.
When you flip a light switch, you know the light will go on. If it doesn’t go on, you have a process for determining why. You change the bulb, fix the switch, or call an electrician. We control how the light works, because we know causes and effects. We can deduce from those relationships other interactions, and we constantly try to make more and better connections between phenomena. The better we are at linking cause and effect, the more effective we are at staving off the brute forces of nature.
When we can’t make a direct connection between cause and effect, we can make guesses and use trial and error to see if we are close. We can isolate small parts of a process to see how the pieces work and make associations from there. Science is the determination of causes and effects so that they can be used to advantage in some way.
Some systems present more difficulty, because there may be no direct link between cause and effect. With a multitude of simultaneous inputs or the complexity of the interactions and feedback responses, particular effects might be masked by others. In complex adaptive systems, such as human society or a biological organism, inputs might actually change the system itself, so that the output is unpredictable or removed from the input in time or space.
When we can’t relate cause and effect, however, that doesn’t mean that the connection doesn’t exist. We just haven’t found it yet. A turbulent surf would be almost impossible to strictly identify all causes and relate them to all effects. We can, however, isolate waves in experiments to determine how one behaves, how two interact, and, using that knowledge, understand the confused crashing of waves onto the shore. We know there are causes for all effects. With living organisms, we can figure out what usually works. Sometimes close enough is close enough for good results.
Social constructions do actually exist, however. Government is an example. While it is real, what we call government is simply individual people who have, through some social mechanism, been given authority to make decisions for masses of other people. They, as individuals, with their own assumptions and self-interest, use buildings, facilities, and equipment to accomplish the things they decide to do, using their understanding of cause and effect.
There seems to be a growing number of confused people who can’t make the distinction when it is in their self-interest not to. People who say there is no reality, however, expect their car to start when they turn the key. They still decide not to drive into a giant oak tree at 100 mile per hour. They still eat food when they are hungry and persist in all necessary bodily functions. Regardless of what they say, they act as though they really believe that reality is real. Welcome to the club.