Cum negante principia nequit disputar
"It is impossible to discuss with anyone who denies principles"
Our knowledge consists of judgments, connected and coordinated with one another. The progressive life of the mind moves by a regular process in which judgments are built upon other judgments, so that the judgment is the principal and central act of the mind (II, D). Amongst these mental enunciations there are some which play a capital role in the life of the mind. They rule not only its psychological development, but also its epistemological and logical functioning, and therefore they deserve our special attention. We call them the directing principles of knowledge. To this class belong:
the principle of contradiction (a thing cannot both be and not be);
the principle of identity (that which is, is; being is equal to itself);
the principle excluded middle (there is no middle term between being and non-being);
the principle of sufficient reason (being is endowed with all the elements without which it could not be);
the principle of totality (the whole is equal to the sum of its parts);
the principle of efficient causality (non-necessary being exists by the influence of a being other than itself).
There are many others. All form one long series, in close connection with the principle of contradiction, of which they all express different elementary phases or applications.