In "A Treatise of Human Nature", Hume pointed out that although we can observe that one thing follows another, we cannot observe the connection between any two things. According to his skeptical theory of knowledge, we can only believe in knowledge based on our observations. Hume argued that our concept of cause and effect is nothing more than our expectation that one thing will follow another. "We have no way of knowing the relationship between cause and effect. We can only know that certain things are always connected together, and these things have never been separated in past experience. We cannot see through the connection behind these things. Reason is that we can only observe these things themselves, and find that these things are always classified in our imagination through a constant connection." Therefore, we cannot say that one thing creates another thing. , all we know is that one thing may be related to another thing. Hume proposed the word "constant conjunction" here. Constant conjunction means that when we see that something always "causes" another thing, what we see is actually that one thing always "constantly" with another thing. Therefore, we have no reason to believe that one thing actually causes another thing, and the two things may not always be "connected" in the future. The reason why we believe in causation is not because causation is the essence of nature. It's the psychological habits and human nature we've developed that have little impact. Hume plans to see if the book will be well-received, and if so he will continue to write about politics and respond to criticism. However, this book did not receive any attention (as Hume himself said: "The media's response to this book was a dead silence, not even the slightest explanation to the enthusiastic readership"), so Hume No further writing plans for the second half.