For any logical inference, you need at the outset a definite axiom which is assumed without justification. It must, otherwise, be included in a larger category, such that definition of any genus requires a more comprehensive genus, in relation to which former is its kind. As the ladder is climbed every larger category become more impoverished, therefore at the top there is not any defining genus. The most comprehensive category is Being, which in fact does not contain any determination in it, hence has not capacity of explanation. There is no pure concept which gets its meaning only by itself.
Every concept is mediated by other concepts.
As a result, formal logic gets into trouble both horizontally and vertically. Similarly it can be said that dialectical logic is also, but more developed tool. Dialectical Logic Contrary to formal logic, the law of dialectical logic is that everything is mediated therefore everything is itself and at the same time not itself. For Hegel, identity is the unity of different terms, and difference is the difference of united terms, namely, negation of negation. To claim for two different things are the same at one and the same moment is contradiction.