Democritus’ fr. 117 says: We know nothing in reality; for truth is in the depth. Obviously it is an enthymeme (according to later terminology) or an incomplete syllogism. Two questions arise: 1.What is the missing proposition, so that the transition from Truth is in the depth to the conclusion We know nothing in reality is justified? 2. Are the two parts of this fragment compatible? Strictly speaking they seem to be contradictory. In my article I tried to fill in the missing premiss and find a link between the two extant propositions by giving: 1. Some clarification of Democritean terminology, 2. A corresponding translation of other fragments referring to the Democritean epistemology, 3. A comparison of their meaning. Fragments related to the discussion are 6-10 and 11, 125. Between these two groups there is an apparent incompatibility ; from the former an impression of skepticism is unavoidable; in the latter a cooperation of senses and mind is discussed and a possibility (for Man) to true knowledge is seen clearly. I put forward the argument: 1. Fragments 6-10 express only sensory skepticism. 2. Fragments 11 and 125 say: truth is accessible to Man by cooperation of senses and mind; the cooperation of both sides in necessary in our knowing activity. 3. The two groups are compatible; a sensory skepticism does not preclude a general epistemological optimism; on the contrary it opens up a critical way of thinking about the limits of human capacity. 4. Fr. 117 is better understandable as a link between the two groups. Now we can proceed to fill in the empty space in the reference we failed to understand in the beginning: 1. Truth is in the depth (the fundamental belief of Democritus in accordance to the atomic theory). 2. By our sensory equipment we apprehend only appearances (phenomena, the surface of reality, secondary qualities). Therefore 3.We apprehend no truth (primary qualities, reality, atoms). Two final remarks are: 1. Democritus’ epistemology is a supplement to the atomic theory and derives from it. 2. If I had to classify it in modern terms, I would say that most fitting term for it is «critica philosophy».