The second concept Wiener has to establish is that of entropy.
Probability is a mathematical concept, coming from statistics.
Entropy comes from physics. That is, in any given situation, less organization, more chaos, is overwhelmingly more probable than tighter organization or more order. It applies however, to a closed system, to something that is an isolated whole, not just a part. And one of them is life, especially human life, which in a universe moving inexorably towards chaos moves towards increased order.