The author’s reference to Democritus’s teaching on knowledge development is explained by the need to highlight such ideas, the enduring significance of which for subsequent theories of learning is confirmed by time. It is revealed that Democritus was one of the first to identify two types of knowledge development (sensual and theoretical) and the corresponding types of knowledge. For subsequent theories of learning, the following ideas by Democritus are significant: 1) knowledge development goes in depth from sensory to theoretical knowledge; 2) knowledge developed through the senses cannot provide a stable basis for recognizing the truth; 3) knowledge developed through thought is reliable in estimating the truth and recognizing it. The most important aspects of Democritus’ thoughts are identified, which in the field of modern terminology can be called humanitarian: theoretical thinking (operating with concepts, etc.) educates a person as a thinking being; when raising a thinking student, it is necessary to take in consideration the types of knowledge development.