Under the name "logic" that appears to be the first Zeno introduced, the Stoics understand in a broad sense all the research on the internal laws and external expressions of human thinking, so it is divided into two parts - rhetoric and dialectics. Rhetoric has three titles concerning preparation, proper exposure, and correct speech taking in terms of the area of knowledge that is relevant to each individual case. The dialectics are also divided into three parts: logic, in the narrow sense of the word, studying concepts, judgments, and mummies; the theory of cognition and grammar, for whose scientific reasoning and development the Stoics have great merits. They first studied the issue of the origin of language, which they perceived not as the result of a deliberate individual discovery but as the product of spontaneous and unconscious communication with humans.
In contrast to Plato and Aristotle, who, in their teaching of knowledge, proceeded from broad rational preconditions, the Stoics were empiricists. Knowledge, according to them, should start from the perception of the single specific things. At the birth of man his soul is like a pure papyrus (harti eirgos eis apographin), later expressed in the expression "pure board", which is written by perceiving things. All our imaginations / fantazia /, forming the content of our knowledge, arise from sensations, received by the senses. They are like prints on the soul that make changes in it. Also for our own inner states and manifestations we receive information through sensations. Since, according to the Stoics and everything in-house, it is expressed in material processes, they do not in fact make a distinction between internal and external sensations.
From sensations and perceptions, images come from memories, and the unification of such ideas gives our cognitive experience. By comparing and plumbing our soul from separate ideas, it generates common concepts (koinai, ennoiai). These general concepts, according to the Stoics, have before any concrete acquaintance as their preconceptions, their preliminary convictions, which are innate to him and which, in the context of the actual acquaintance, are reborn. That is why the Stoics call them pretensions, premonitions. Through the further formation of a concept and through evidence, a science is created, which differs from the ordinary opinion / in that it is a collection of unshakable, strictly established and lasting beliefs and concepts /.
Since all concepts and concepts are acquired through perceptions, the main gonaeological question arises: whether perceptions are certainly consistent with perceived objects. The Stoics teach that there are such perceptions that, with absolute necessity and obviousness, compel us to assume that they correspond to reality. They have internal axiomacy (anargeia) and Zeno calls them "cognitive", conceived ideas / sigkatatidesdai /. These, in fact, only obvious, equivocal concepts, according to the Stoics, form the basic criterion of truth. Truth is, according to them, also what is universal conviction. So, they accepted the testimony of the so-called common common sense.