Hume is skeptical about the fundamental problem of epistemology: "Where does the feeling impression come from?" . He claimed: "As for the impressions produced by the senses, in my opinion, their ultimate reason can't be explained by human reason. We can never be sure whether those impressions are directly produced by the object, by the creative ability of the mind or by our creator "(Theory of Human Nature, p.10/page). This is the basic view of Hume's skepticism, that is, agnosticism, which has no function of material objects and no function of mind or God.

Although Hume tried his best to avoid answering questions about the origin of matter or spirit, his skeptical philosophy did not surpass materialism and idealism, but only wavered between them. When Hume admitted that all consciousness depends on the organs of the body, and this feeling has its natural physical reasons, he was closer to materialism. When he separated perceptual perception from the material world and asserted that "the only thing that really knows is perception" and "any conclusion about the existence of an object can never be formed by the existence of perception or any of its properties", he fell into subjective idealism.

Hume called his philosophy "moderate skepticism" to distinguish it from the "excessive skepticism" of Pyrrho in ancient Greece. Although Hume's skepticism limited knowledge, he did not completely deny it. On the contrary, he affirmed that the human mind was endowed with some cognitive ability, and thus put forward the task of studying human rational ability.

Hume believes that causal knowledge based on experience constitutes most of human knowledge and is the source of all human behaviors. Therefore, he focused on the issue of causality. According to skepticism, he does not admit any conclusion about objective regularity, but only focuses on the causal relationship between various phenomena and concepts. He pointed out that the relationship between cause and effect is continuous, but "after this" is not "therefore". The basic condition of the concept of causal connection lies in inevitable connection, that is to say, every cause must have a cause, and every cause must have a result. The discovery of this causal relationship depends not on reason, but on experience. However, a single experience is not enough, but also depends on experience. However, a single experience is not enough to form the concept of causality. This concept can only be formed when similar phenomena are repeated many times or often gathered together. In his explanation of causality, he thinks that causality exists only in the heart, not in things, and falls into subjective idealism.