Philosophers and sundry pseudo-intellectual academics, psychologists, religionists, and mystics can tell you with absolute certainty that certain knowledge is not possible. Human beings cannot do anything they must do to live without knowledge. If knowledge is not possible, human life is not possible.
Fortunately, by the time one is an adult, one knows literally millions of things, and most of them are certain.
Things you know for certain:
What you are currently experiencing.
What you are currently doing.
Where you currently are.
What you are seeing.
What you are hearing.
What you are feeling.
What you are tasting.
What you are smelling.
That you are reading.
That you are thinking.
That you are awake.
That you exist.
That you are conscious.
That you are conscious of something.
That it is day (or night).
That is is raining (or it isn't).
That it is warm (or cold).
What your nose, ears, eyes, mouth, arms, hands, legs, feet and chest are.
What and where the bedroom, kitchen, bathroom, doors, and windows are.
What clothes are.
What pants, shirts, socks, and underwear are.
What a dress, slacks, blouse, sweater, panties, and bra are.
What water is.
What food is.
What bacon, eggs, tomatoes, potatoes, onions, grapes, and apples are.
What meat, flour, cake, cookies, crackers, and snacks are.
What hamburgers, hot dogs, tacos, french fries, and ice cream are.
What a knife, fork, spoon, plate, bowl, glass, cup, and saucer are.
What a refrigerator, stove, dishwasher, washing machine, and clothes dryer are.
What a sink, bathtub, toilet, and shower are.
What cars, trucks, boats, planes, trains, and motorcycles are.
What a book, a TV, a table, a chair, a desk, a lamp, and a computer are.
What a cat, dog, cow, chicken, pigeon, goat, sheep, pig, and horse are.
What clocks and calendars are.
What time, day, month, and year it is.
What a building, home, store, restaurant, factory, and police station are.
What a pond, lake, brook, river, hill, mountain, valley, and cliff are.
What trees, bushes, weeds, flowers, grasses, and funguses are.
What stars, the moon, and the sun are.
What counting, addition, subtraction, multiplication, and division are.
What a party, a team, a show, a circus, a ceremony, and a speech are.
When something is alive.
When something is dead.
When the lights, TV, water, gas, or radio are on or off.
That electric lighting is possible.
That heavier than air flight is possible.
That refrigeration is possible.
That sound and pictures can be transmitted electronically anywhere in the world.
That getting to the moon is possible.
That X-rays are possible.
That anaesthesia (painless surgery) is possible.
That bacteria cause disease.
That vaccination is possible.
That lasers are possible.
[The last ten things were all unknown or declared impossible by philosophers and academics until 300 years ago. Now they are all certain.]
You know a language and how to read, understand, and write it. (Else you would not be reading and thinking about these things.)
You know all those things and thousands more as well as hundreds more things about each one of the things you know, and you know them all for certain.
Why Human Beings Must Have Knowledge
Except for human beings, most animals are able to do everything their nature requires them to do to live successfully, often within a few hours or days of their birth. They are able to walk, run, fly or swim, perform their biological functions, find and acquire the kind of food they must eat, prepare whatever shelter they need, mate and raise their young. Human beings are born unable to do anything their nature requires them to do to live as human beings.
A human being cannot do any of the things their nature requires them to do, even to live, much less to live successfully, and until they they have learned those things most of what their life requires must be done for them, and it will take years of learning to be able to live as a human being.
There is nothing your life requires you can have without knowledge. When you are first born it is not your own knowledge that keeps you alive, fed, clothed, sheltered, and safe from the dangers of life, it is the knowledge of those who choose to love and nurture you, but it is still their knowledge of how to provide those things that make your young life possible. As you grow older, more and more of the things your life requires will depend on the knowledge you gain as you grow and mature. By the time you are an adult, how you live depends on your own knowledge.
There is not a single thing a human chooses to do that can be done without knowledge. From the simplest daily routines of life to the most difficult tasks of one's occupation, every action requires knowledge. By the time we are able to dress ourselves and prepare our own meals the enormous amount of knowledge required to perform such tasks is taken for granted, but none of them could be performed if one did not know left from right or front from back, or how to use a can opener, or what a refrigerator is, or what time they had to be at work. Whatever you choose to do with your life from yard work to brain surgery, nothing you do will be possible without knowledge.
That human beings have knowledge is irrefutable. Everything human beings have ever done and produced is the evidence, because none of it would have been possible without knowledge. Just the fact there are human beings is proof they have knowledge, because their existence would be impossible without it.
Certain Does Not Mean Infallible Or Omniscient
The philosophers and mystics that want to convince you certain knowledge is not possible do not mean what you and I mean by certain knowledge.
Certain knowledge does not mean infallible knowledge. It does not mean one can never be mistaken, but that is what the deniers of knowledge mean. The fact that one can make mistakes does not mean they can only make mistakes. Mistakes are always exceptions.
Certain knowledge does not mean omniscient knowledge. It does not mean one knows everything there is to know about everything there is, or even about anything. The fact that one cannot know everything does not mean one knows nothing. If one knew everything there would be nothing else to learn. No new experience or discovery would be possible.
What Is Known
All there is to know is what exists: all the physical entities one encounters or learns about, all their attributes and characteristics, all their behavior, and their relationships to each other, (material existents), as well as all those things which exist as the product of the human mind, such as language, mathematics, logic, art, literature, the physical sciences, history, geography, philosophy, and superstitions.
One knows what things are. All the things listed as, "things you know for certain," at the beginning of this chapter are things one knows, because they know what they are. Whether physical things (like articles of clothing or food), experiences (like seeing and smelling), or ideas, (like vaccination and refrigeration), one knows what they are because each has its own unique set of characteristics and attributes which describe them. If one did not know what they were, they would not be able to think or talk about them or use them.
Knowing what those things are does not mean knowing all there is to know about any of them, but it does mean one knows something about them, at least enough to identify them and distinguish them from other things. The fact that one knows what they are, together with whatever they know about them is certain knowledge.
What About Mistakes, Ignorance, Superstition, Deception, and Illusion?
Certain knowledge does not mean infallible knowledge, omniscient knowledge, that everyone has any particular knowledge, that all supposed knowledge is true knowledge, or that one can never be deceived.
None of those things—mistakes, ignorance, superstition, deception, or illusion—could be known without certain knowledge. If nothing could be known for certain, there would be nothing but mistakes, ignorance, superstition, deception and illusion. It is the fact that we are usually right that we can tell when we've made a mistake. It is the fact we know what knowledge is that we can recognize when it is lacking. It is the fact that we know what certain objective knowledge is that makes it possible to know what superstition is. It is the fact deception is an exception that it can be detected. If there were no certain knowledge, an illusion could never be discovered. If one did not know what a true (perceived) representation is, there would be no way to know an illusion was not a true representation of reality.
Those Who Deny Certain Knowledge
What motivates any particular individual to deny true knowledge is possible is not important, but it is never innocent. The denial of knowledge is always an excuse for putting over some irrational view that rational knowledge would make impossible, usually some mystic or political ideology that cannot possibly be rationally defended. The most common of their arguments are, "you cannot prove it is not true," and, "you cannot know anything for certain, so it's just your opinion."
One thing you can know for certain is that anyone who tries to convince you what you know cannot be certain is trying to put something over that they know is not true. They just don't want you to know it.