The aim of the present paper is to show how Plato suggested demarcating between knowledge and other kinds of human intellectual activities. The article proposes to distinguish between two ways of such a demarcation. The first, called ‘the external demarcation’, takes place when one differentiates between knowledge and non-knowledge, the rational and non-rational or the reasonable and non-reasonable. The second, called ‘internal’, marks the difference within knowledge itself and could be illustrated by the difference between the so called hard and soft sciences. The analyses lead to the following conclusions. Plato refers to the whole of human intellectual activity as doxa, which is divided into two spheres. The first of them is knowledge proper whose criterion is phronesis. Three other kinds of doxa are derived from knowledge proper: 1 the traditional peri phuse¯os investigation called also sophia; 2 popular doxai concerning virtues; 3 wisdom of the antilogikoi. The difference represents the external demarcation. There may be, however, a difference in the scope of knowledge proper the internal demarcation. If the peri phuse¯os investigators were able to explain the field of values, the result of their investigation could be acknowledged as knowledge, although it would still be characterized as inferior due to its being based on senses. What is interesting about knowledge proper is that it is not firm and reliable but only hypothetical. It does not determine the skeptical reading of the Phaedo but it indicates that Plato has just begun his own philosophical project which is still in progress and the knowledge presented in the dialogue is his first positive suggestion how to solve the problem of demarcation.