Because the real world is a world of ideas. We all know that the current scientific level of mankind can be said to be changing with each passing day and is very developed. According to the Marxist philosophy we learned from middle school politics textbooks, we seem to be able to continue exploring until we exhaust the mysteries of the universe, because the world is materialistic. However, the famous physicist Popper had such a view, saying that "all knowledge possessed by mankind is speculation and hypothesis", which is very shocking. Is it possible that such a highly renowned physicist still believes in absurd idealism?

First of all, we must clarify what kind of idealism Popper held. I believe we all have learned the definition of "idealism" from middle school politics textbooks. It seems very ridiculous, such as "If you close your eyes, the flowers will wither". As long as you feel that there is no cliff in front of you, you will not fall to your death. Is this kind of view idealism? If idealism is really so shallow and absurd, how can it dominate the philosophical world? Even famous philosophers such as Wittgenstein and Heidegger in modern times are so-called "idealists"? In fact, the "idealism" of idealism refers to "the world is a world of facts, not a world of things." The world of things? This sentence comes from Wittgenstein, and I think it is very concise and comprehensive. In other words, the universe we recognize is the universe in our consciousness, which is a "collection of ideas." As for the nature of the universe, we cannot assert: "The universe I recognize is the entire universe." Wittgenstein also has a famous saying: "The boundaries of language are the boundaries of thought." In other words, the knowledge that humans have is all within the scope of what humans can recognize. We can only get as close to the truth as possible, but we can never know the complete truth because it is beyond human capabilities. The universe of "ideas" does not mean that we can give up the exploration of science. On the contrary, it inspires the confidence of scientists to continue exploring the world using what Kant called "reason". Einstein, Popper, Copenhagen, they are all such "rationalists".

To sum up, because the universe is a universe of concepts, the knowledge we have is only hypothesis and speculation.