"Tautology!" you say? Ok, I'll provide as many additional counter-examples as you wish later on. First, let's try to understand what the objective reality even is. Ironically enough, the story begins inside the totally subjective human brain.
The Subjective Brain
|we need this brain|
Are your eyes recording everything visual around you? They are not. The high-definition dynamic image you think you "see" now is really a re-creation of information your brain has collected, with occasional reference points and updates (3 per second or so) gleaned from the optic nerves and from other sensory input. You "see" with your eyes, nose, ears, sense of touch, etc., but you mostly "see" with your brain. Want proof? Ok - here it is. You can see perfectly well in the dark with your eyes closed. You do it every time you dream.
Color can physiologically only be perceived in the fovea, that tiny dense circle of photoreceptors at the center of the eye with which you are reading these letters now. You can't quite make out the words that you aren't looking directly at, can you, because only the fovea has enough "megapixels" to clearly resolve them. It also has the only color receptors in your entire eyeball. The colors that you "see" in your peripheral vision are not being seen directly by you now; they are only the memory of the colors that you saw there before.
Pain is another illusion created entirely by the brain. When you stub your toe, it hurts. The pain feels completely real. That is the power of the brain's capacity to create illusion: it seems completely real because the brain is the only way you have of experiencing anything at all, real or otherwise. In reality, your actual toe feels perfectly fine. It does not experience pain; it has no consciousness with which to make that judgement. The pain exists only in the brain's virtual re-creation of your body, called the body schema. Pain is there only to tell you to watch where you are going next time, but it can become a habit. Few people are aware that they can switch it off at will, and some people are even offended when told they have this ability.
Incidentally, there are numerous crazy ways that "body schema" makes itself known to us: through bulimia - the belief that a thin body is actually fat, "phantom limb syndrome" of amputees, "foreign limb syndrome" where people feel so uncomfortable with a particular arm or leg that they cut it off, and other hallucinations about the body.
The brain creates your own personalized version of reality in other ways, too, that people are not often naturally aware of. The filtration system at the center of your brain, called the Reticular Activating System or RAS, only lets through a tiny proportion of the information it receives every second, with the "tuning" of this filter under the control of your unconscious mind. It thinks it knows what trickle of items are important for you to be consciously aware of, and "protects" you from a veritable firehose of sensory input it deems irrelevant. Another secret of the brain that people are often disturbed to be told about is the fact that you can, with practice, consciously "tune" your RAS to the things that you willfully decide are important for you.
With the brain's virtual reality engine the only link your consciousness has to the "outside" world, people can be forgiven for concluding that there is no reality outside, and that "all knowledge is subjective." But that doesn't make them any less wrong.
The Objective UniverseLife in this world is a never-ending experiment in which we probe, test, examine and manipulate reality. I can paint a wall blue, and wait. Sooner or later another consciousness will bring itself to my attention and insist that the wall over there has been painted blue. Yup. I did that. I manipulated Objective Reality. I wouldn't be able to do that if it didn't exist.
But distinguishing between reality and illusion with any certainty does take discipline and effort. One has to disbelieve almost everything, or at least only "conditionally accept" what your brain tells you, pending independent verification. Through this process, you can come to know that there is an objective universe out there, consisting of matter and energy.
There exists, for example, particles designated "electrons" which have a mass (at rest) of between 9.109382910x10-31 kg and 9.109382918x10-31 kg, and this range is 99.99% certain. They also repel other electrons with a force that is inversely proportional to the square of the distance between them. This is an example of what we call a "law" of the objective reality nicknamed "Nature."
Disciplined humans peeling back the biases and re-creations of the subjective mind have discovered this information independently all over the world. Also, there is strong evidence that these electrons exist in even the most distant galaxies as far away and as far back in time as our machines, which have no subjective mind or consciousness, can detect.
Some subjective minds have labelled this discipline "science" meaning "knowledge," and have dismissed it as just another illusion created by the subjective mind. While the discipline may be a human invention, the knowledge uncovered is not invention but discovery. You may travel to a city in a car that belongs to you, but the city it takes you to does not therefore also belong to you.
People who insist that there is no objective knowledge are only speaking for themselves in their own undisciplined, disinterested ignorance. But in a sense, they are right: there is no objective knowledge that they are aware of.